Musnad Ahmad 26:321

Abu Ahmad narrated to us, Isa ibn Dinar narrated to us, from his father, from Amr ibn al-Harith ibn Abi Dirar al-Khuza'i, who said: I heard Abdullah ibn Mas'ud saying: "I did not fast twenty-nine days with the Messenger of Allah, peace and blessings of Allah be upon him, more than I fasted thirty days with him."