Musnad Ahmad 26:750

Yahya ibn Zakariya ibn Abi Za'idah narrated to us, he said: Isa ibn Dinar narrated to me, from his father, from Amr ibn al-Harith ibn Abi Dirar, from Ibn Mas'ud, who said: "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."