Musnad Ahmad 26:234

Hassan ibn Musa narrated to us, Hammad ibn Zayd narrated to us, from al-Mujalid, from al-Sha'bi, from Masruq, who said: We were sitting with Abdullah ibn Mas'ud while he was teaching us the Quran. A man said to him: "O Abu Abd al-Rahman, did you ask the Messenger of Allah, may Allah bless him and grant him peace, how many caliphs this nation will have?" Abdullah said: "No one has asked me about this since I came to Iraq before you." Then he said: "Yes, we did ask the Messenger of Allah, may Allah bless him and grant him peace, and he said: 'Twelve, like the number of the chieftains of the Children of Israel.'"