Mohammad bin Hasan bin Ahmad narrated from Mohammad bin Husain, from Ibrahim bin Hashem, from Mohammad bin Sinan, from Ziyad bin Munthir, from Saeed bin Turayf, from Asbagh bin Nobata, from Ibn Abbas, who said:
I heard the MESSENGER OF ALLAH (SWT) say, “O people! Understand that ALLAH (SWT) has made a gate to Himself for you. If you enter from it you will be safe from Hell and the great terror.”
Then Abu Saeed Al-Khodri stood up and said, “Guide us to this door so we recognize it.”
The PROPHET (SAWW) replied: It is ALI BIN ABI TALEB (ASWS), the Master of Successors, the COMMANDER OF THE BELIEVERS (ASWS), brother of the MESSENGER OF ALLAH (SWT), and ALLAH (SWT)’s caliph on all people.
O people! Those of you who love to hold on to the firmest handle that never breaks off, hold on to ALI BIN ABI TALEB (ASWS)’s Wilayat. His Wilayat is my Wilayat, and obeying him is obeying me.
O people! Those of you who want to know who ALLAH (SWT)’s decisive proof is after me, know that it is ALI BIN ABI TALEB (ASWS).
O people! Those of you who want to follow ALLAH (SWT) and His Messenger, follow ALI BIN ABI TALEB (ASWS) and the Imams from my family because they are the keepers of my knowledge.
Then Jabir bin Abdillah Al-Ansari asked, “How many Imams are there?” The PROPHET (SAWW) replied:
O Jabir! May ALLAH (SWT) have mercy on you. You have asked me about the entire Islam. Their number is the number of the months.
"Verily, the number of months with ALLAH (SWT) is twelve months in ALLAH (SWT)'s book since the day He created the heavens and the earth"(9:36)
Their number is equal to the number of rivers that ALLAH (SWT) created with a blast when Musa bin Imran (a.s.) hit the rock with his staff, which is twelve. Their number is equal to the number of the chiefs of Bani Israel, as ALLAH (SWT) says in the Qur'an,
“We raised from among them twelve chiefs" (5:12)
Therefore Jabir, the Imams are twelve. The first one is ALI BIN ABI TALEB (ASWS) and the last one is MAHDI AL-QAEM (AFFS).
(Ref: Bihar Volume 36 Page 263 Hadees No.84)