There is an interesting tradition in Sahih al-Bukhari as well as Sahih Muslim, in which the Prophet (PBUH&HF) said the following:

Sahih al-Bukhari Hadith: 9.422

Narrated Abu Sa'id al-Khudri:

The Prophet said, "You will follow the ways of those nations who were before you, span by span and cubit by cubit (i.e., inch by inch) so much so that even if they entered a hole of a mastigure (lizard), you would follow them." We said, "O Allah's Apostle! (Do you mean) the Jews and the Christians?" He said, "Whom else?"

As the above tradition in Sahih al-Bukhari confirms, the Prophet stated that the history of the Children of Israel will be repeated for Muslims. In fact, Quran has mentioned the stories of the Children of Israel to give us a way to understand the true history of Islam itself. There are many striking similarities in this regard written in Quran including the similarities of the leaders and the similarities of the people. I just mention few of them here. Allah, to whom belong Might and Majesty, said:

"Surely Allah aforetime took a covenant from the Children of Israel and We appointed twelve leaders among them" (Quran 5:12)

وَلَقَدْ أَخَذَ اللَّهُ مِيثَاقَ بَنِي إِسْرَائِيلَ وَبَعَثْنَا مِنْهُمُ اثْنَي عَشَرَ نَقِيبًا

Who are those twelve leaders among the Children of Muhammad (PBUH&HF)?

Allah, Exalted, also said:

"And when Moses prayed for water for his (thirsty) people; We said: 'Strike the rock with your staff.' Then from there surged forth twelve springs so that each group knew its own place to drink." (Quran 2:60).

: | وَإِذِ اسْتَسْقَى مُوسَى لِقَوْمِهِ فَقُلْنَا اضْرِبْ بِعَصَاكَ الْحَجَرَ فَانْفَجَرَتْ مِنْهُ اثْنَتَا عَشْرَةَ عَينًا قَدْ عَلِمَ كُلُّ أُنَاسٍ مَشْرَبَهُمْ

Who are those twelve springs (streams) of knowledge who quench the thirst of Muslims to the end of the world so that each generation is utilized with one of them? He, Exalted He is, also said:

"We divided them into twelve nations. We directed Moses by revelation when his (thirsty) people asked him for water: 'Strike the rock with your staff;' out of it there gushed forth twelve springs so that each group knew its own place for water. We gave them the shade of clouds and sent down to them manna and quails (saying): 'Eat of the good things We have provided for you,' (but they rebelled.) To Us they did no harm but they harmed their own souls." (Quran 7:160).

Verily those who did not follow those twelve leaders, harmed non but themselves. The above verse suggests that the nation of the Prophet Muhammad (PBUH&HF) in the span of history (after his demise till the day of judgment) are divided into 12 intervals of time which coincides with one Imam who is assigned as leader for them. In the next verse Allah said:

"And when it was said unto them: 'Dwell in this Town and eat therein as you wish, and say Repentance, and enter the Gate (of the city) in a posture of humility, in order that We forgive you your sins and increase (the portion of) those who do good.'" (Quran 7:161)

or:

"And when We said: 'Enter this Town and eat of the plenty therein as you wish; but enter the Gate (of the City) with humility in posture and in words so that We forgive you your sins and increase (the portion of) those who do good.'" (Quran 2:58).

The Gate in the above verses has a striking similarity with one of the attributes of Imam Ali (AS) mentioned by our Prophet (PBUH&HF) that is: "The Gate of the City of Knowledge."

The Messenger of Allah said: "I am the City of Knowledge, and Ali is its Gate. So whoever intends to enter the City and the Wisdom, he should enter from its Gate."

انا مدينه العلم و علي بابها و من اراد المدينه والحكمه فلياتها من بابها

Sunni references:

- Sahih al-Tirmidhi, v5, pp 201,637

- al-Mustadrak, by al-Hakim, v3, pp 126-127,226

- Fadha'il al-Sahaba, by Ahmad Ibn Hanbal, v2, p635, Tradition #1081

and many more...

Furthermore the following Prophetic tradition gives the utmost similarity with the above two verses:

The Messenger of Allah said: "My Ahlul-Bait are like the Gate of Repentance of the Children of Israel; whoever entered therein was forgiven."

انما مثل اهل بيتي فيكم مثل باب حطه في بني اسرائيل...

Sunni References:

- Majma' al-Zawa'id, by al-Haithami, v9, p168.

- al-Awsat, by al-Tabarani, Tradition #18

- Arba'in, by al-Nabahani, p216

- A fairly similar tradition reported by al-Darqunti and by Ibn Hajar al-Haithami in al-Sawa'iq al-Muhriqa, Ch. 9, section 2, p193, in which the Prophet (PBUH&HF) said: "Ali is the Gate of Repentance, whoever entered therein was a believer and whoever went out was an unbeliever."

Also Allah (SWT) stated in Quran:

Lo! The number of the months with Allah is twelve which is Allah's ordinance when He created the heavens and the earth. Four of them are sacred. That is the firm religion. So wrong not your souls in them. (Quran 9:36)

إِنَّ عِدَّةَ الشُّهُورِ عِنْدَ اللَّهِ اثْنَا عَشَرَ شَهْرًا فِي كِتَابِ اللَّهِ

In connection with the above verse of Quran, it would be nice to look at the following tradition in Sahih al-Bukhari:

Sahih al-Bukhari Hadith: 5.688

Narrated Abu Bakra:

The Prophet said, "Time has taken its original shape which it had when Allah created the Heavens and the Earth. The year is of twelve months, four of which are sacred... Surely, you will meet your Lord, and He will ask you about your deeds. Beware! Do not become infidels after me, cutting the throats of one another. It is incumbent on those who are present to convey this message (of mine) to those who are absent. May be that some of those to whom it will be conveyed will understand it better than those who have actually heard it." He (i.e. Prophet) then added twice, "No doubt! Haven't I conveyed (Allah's Message) to you?"

Now one may ask that what was in the above message that could not be understood by the companions who were listening to the speech of the Prophet during his last pilgrimage in Mecca? (For timing see also Sahih al-Bukhari Hadith: 2.798). The Message of the Prophet (PBUH&HF) had twofold meaning. The obvious meaning is that the number of months are twelve and four months, namely Dhul-Qa'da, Dhul-Hajja, Muharram, and Rajab are sacred months. In fact, these months were believed to be sacred even before Islam. So there was nothing in this message that the audience could not comprehend.

Moreover, the fact the above mentioned sacred months of the year were accepted by the Jews and Christians, makes it clear that these months can not be "the firm religion" as mentioned in the verse. So one should look for a more delicate meaning.

The other meaning (as interpreted by Ahlul-Bait) is that the Prophet in his last pilgrimage (less than three month before his demise) wanted to convey that he will be followed by twelve Imams and people should not wrong their souls by disobeying them in their leadership periods. Among these twelve Imams, four have a sacred name, i.e., "Ali", which is derived from the name of Allah. In fact, four Imams of Ahlul-Bait (peace be upon them all) have been named "Ali".

In the Sira of Ibn Hisham, there is an extra sentence of the Messenger of Allah which is, in fact, the verse of Quran. The Messenger of Allah (PBUH&HF) said:

"'Postponement of a sacred month is only an excess of disbelief whereby those who disbelieve are misled; they allow it one year and forbid it another year that they may make up the number of months which God has hallowed so that they permit what God has forbidden (Quran 9:37), and forbid what God has allowed. Time has completed its cycle and is as it was on the day that God created the heavens and the earth. The number of months with God is twelve. Four of them are sacred."

Sunni references:

- Sira, by Ibn Hisham at the end of Chapter of "Farewell Pilgrimage", p968

- The Life of Muhammad (English translation of Sira Ibn Hisham), Translated into English by A. Guillaume, 1955 Edition, London, p651

Postponement of the sacred month is delay in accepting their leadership and as the messenger of Allah said, those who disbelieve in their leadership will be misled. They permit what God has forbidden, and forbid what God has allowed. They try to make up the twelve Imams by that which Allah has not honored.

The fact that some sects departed from the main body of the Shi'a in the history was because they only accepted first few Imams and rejected the rest. It is interesting to know that he who acknowledged all four "Ali" among the Imams, he has acknowledged all the twelve Imams, since there did not remain any sect believing in these four Imams and rejecting others. In a tradition on the authority of Jabir (RA), Imam Muhammad al-Baqir, the fifth Imam of Ahlul-Bait, interprets the above verse which is as follows:

Jabir said: I asked Imam Muhammad al-Baqir about the meaning of the verse: "Verily the number of the months … (9:36)." He (AS) breathed long (out of sorrow) and said: "O Jabir, The 'year' is my grandfather, the messenger of Allah (PBUH&HF), and his family are its 'months' who are the twelve Imams, and are … (naming the Imams one by one). They are the Proofs of Allah on His creation, and Trusties of His revelations and His knowledge. And Those of 'four sacred who are the firm religion' are those four who have the same name, and are Ali, the Commander of Faithful (AS), and my father Ali Ibn al-Husain (AS), and later Ali Ibn Musa (AS), and Ali Ibn Muhammad (AS). Thus acknowledging these four is 'the firm religion, so wrong not your souls in them' and believe in all of them to be guided."

Shi'i reference: Kitab al-Ghaiba, by Shaikh Tusi