Musnad Ahmad 1007:25

Muhammad ibn Ja'far narrated to us, Sa'id narrated to us, from Qatadah, from Ghaylan ibn Jarir, from Abdullah ibn Ma'bad al-Zamani, from Abu Qatadah al-Ansari, that a Bedouin asked the Messenger of Allah, may Allah bless him and grant him peace, about his fasting. He mentioned the hadith, except that he said: "Fasting on Monday?" He said: "That is a day in which I was born, and in it revelation was sent down upon me."