Abu Dawud Book 7, Hadith Number 2430.
Chapter : Not known.
Narated By Usamah ibn Zayd : The client of Usamah ibn Zayd said that he went along with Usamah to Wadi al-Qura in pursuit of his camels. He would fast on Monday and Thursday. His client said to him: Why do you fast on Monday and Thursday, while you are an old man? He said: The Prophet of Allah (PBUH) used to fast on Monday and Thursday. When he was asked about it, he said: The works of the servants (of Allah) are presented (to Allah) on Monday and Thursday.