Musnad Ahmad 1007:34

'Abd al-Rahman ibn Mahdi narrated to us, from Ghaylan ibn Jarir, from 'Abdullah ibn Ma'bad, from Abu Qatadah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) was asked about fasting on Monday. He said: "In it I was born, and in it revelation was sent down to me."