Musnad Ahmad 27:532

Hisham ibn Sa'id narrated to us, Mu'awiyah ibn Sallam narrated to us, I heard Yahya ibn Abi Kathir informing that Abu Salamah informed him, from Abdullah ibn Umar, that he heard him saying: I heard the Messenger of Allah, peace and blessings of Allah be upon him, saying: "The month is twenty-nine."