Musnad Ahmad 26:309

Abu al-Nadr narrated to us, Abu Aqil narrated to us, Mujalid narrated to us, from al-Sha'bi, from Masruq, who said: We were sitting with Abdullah in the mosque while he was teaching us, when a man came to him and said: "O Ibn Mas'ud, did your Prophet tell you how many caliphs there will be after him?" He said: "Yes, the number of the chieftains (nuqaba') of the Children of Israel."