Musnad Ahmad 26:659

Waki’ narrated to us, ‘Isa ibn Dinar, the freed slave of Khuza’ah, narrated to us, from his father, from ‘Amr ibn al-Harith ibn al-Mustaliq, from Ibn Mas’ud, who said: "We did not fast Ramadan during the time of the Messenger of Allah, may Allah bless him and grant him peace, for twenty-nine days more often than we fasted it for thirty."