Musnad Abu Ya'la al-Mawsili 210:16

'Abd al-A'la ibn Hammad narrated to us, he said: Hammad informed us, from 'Asim, from Sawa', the brother of Mughith, from Hafsah, the wife of the Prophet (may Allah bless him and grant him peace): That the Prophet (may Allah bless him and grant him peace) used to fast three days of the month: Monday, Thursday, and the Monday of the following week.