Musnad Abu Ya'la al-Mawsili 152:254

Abu Khaythama narrated to us, Yunus ibn Muhammad narrated to us, Hammad ibn Zayd narrated to us, from Mujalid, from al-Sha'bi, from Masruq, who said: We were sitting with Ibn Mas'ud one night 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 this nation will have?" Ibn Mas'ud said, "No one has asked me since I arrived in Iraq before you." He said, "Yes, we asked the Messenger of Allah (may Allah bless him and grant him peace), and he said: 'Twelve, the number of the chieftains of the Children of Israel.'"