The freed slave of Usamah bin Zayd narrates that Sayyiduna Usamah bin Zayd (may Allah be pleased with him) had property in Wadi al-Qura; he used to ride out to it, and he used to fast on Mondays and Thursdays. I said to him: "Do you fast when you have become old and weak?" He said: "I saw the Messenger of Allah (peace and blessings be upon him) fasting on Mondays and Thursdays, so I said: 'O Messenger of Allah (peace and blessings be upon him), do you fast on Mondays and Thursdays?' He said: 'Indeed, deeds are presented on Mondays and Thursdays.'"
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8435