Musnad Ahmad 941:74

Isma'il narrated to us, Hisham al-Dastawa'i informed us, Yahya ibn Abi Kathir narrated to us, from 'Umar ibn al-Hakam ibn Thawban, that a freed slave of Qudamah narrated to him, that a freed slave of Usamah narrated to him, that Usamah ibn Zayd used to go out to his property in Wadi al-Qura and would fast on Mondays and Thursdays. I said to him: "Why do you fast while traveling, when 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, you fast on Mondays and Thursdays.' He said: 'Indeed, deeds are presented on Monday and Thursday.'"