Musnad Abu Ya'la al-Mawsili 151:65

Shayban ibn Farrukh narrated to us, Hammad—meaning ibn Zayd—narrated to us, from Mujalid, from al-Sha'bi, from Masruq, who said: We were sitting with Abdullah [ibn Mas'ud] after the Maghrib prayer, and he was teaching us the Quran. A man asked him, "O Abu Abd al-Rahman, did you ask the Messenger of Allah (may Allah bless him and grant him peace) how many caliphs will rule this nation?" Ibn Mas'ud said, "No one has asked me about this since I came to Iraq before you." He said, "Yes, I asked the Messenger of Allah (may Allah bless him and grant him peace), and he said: 'Twelve, like the chieftains of the Children of Israel.'"