Musnad Ahmad 26:290

Muhammad ibn Sabiq narrated to us, 'Isa ibn Dinar narrated to us, my father narrated to me, that he heard 'Amr ibn al-Harith saying: I heard 'Abdullah ibn Mas'ud saying: "I did not fast twenty-nine days with the Prophet, may Allah bless him and grant him peace, more often than I fasted thirty days with him."