Musnad Ahmad 941:3

'Affan narrated to us, Aban narrated to us, Yahya ibn Abi Kathir narrated to us, 'Umar ibn al-Hakam narrated to me, from the freed slave of Qudamah ibn Maz'un, from the freed slave of Usamah ibn Zayd, that he went with Usamah to Wadi al-Qura to seek some property of his, and he used to fast on Monday and Thursday. His freed slave said to him: "Why do you fast on Monday and Thursday, while you are an old man who has become frail?" He said: "Indeed, the Messenger of Allah (peace and blessings of Allah be upon him) used to fast on Monday and Thursday. He was asked about that, and he said: 'Indeed, the deeds of the people are presented on Monday and Thursday.'"