Musnad Ahmad 941:39

Isma'il narrated to us, Hisham—meaning al-Dastawa'i—narrated to us, Yahya ibn Abi Kathir narrated to us, from 'Umar ibn al-Hakam ibn Thawban, that a freed slave of Qudamah ibn Maz'un narrated to him that a freed slave of Usamah ibn Zayd narrated to him, that Usamah ibn Zayd used to go out to property he had in Wadi al-Qura and would fast on Mondays and Thursdays. I said to him: "Why do you fast while traveling, even though you have grown old and become frail?" He said: "Indeed, the Messenger of Allah (peace and blessings of Allah be upon him) used to fast on Mondays and Thursdays." I said: "O Messenger of Allah, why do you fast on Mondays and Thursdays?" He said: "Indeed, deeds are presented on Monday and Thursday."