Musnad Abu Ya'la al-Mawsili 210:28

Abu Khaythama narrated to us, Rawh ibn ‘Ubadah narrated to us, Hammad narrated to us, from ‘Asim ibn Bahdalah, from Suwa’ from Hafsah: That the Prophet (peace and blessings of Allah be upon him) used to fast three days of the month: Monday, Thursday, and Monday of the following week.