Musnad Ahmad 26:229

Abu al-Mundhir narrated to us, 'Isa ibn Dinar al-Khuza'i narrated to us, he said: My father narrated to me that he heard 'Amr ibn al-Harith al-Khuza'i saying: I heard 'Abdullah ibn Mas'ud saying: "I did not fast twenty-nine days with the Messenger of Allah, may Allah bless him and grant him peace, more than I fasted thirty days with him."