مُسْنَدُ أَحْمَدَ
Musnad Ahmad 3758
Musnad of Abdullah bin Mas'ood
It was narrated that Masrooq said: We were sitting with 'Abdullah bin Mas'ood and he was teaching us the Qur'an. A man said to him: O Abu ‘Abdur-Rahman, did you ask the Messenger of Allah (ﷺ) how many caliphs this ummah would have? 'Abdullah said: Nobody has asked me about that before you, since I came to Iraq. Then he said: Yes, we asked The Messenger of Allah (ﷺ) (about that) and he said: “Twelve, like the number of the leaders of the Children of Israel."
Musnad Ahmad 3781Its isnad is da'eef because Mujalid - bin Sa'eed al-Hamdani