Ishaq bin Rahwaya إسحاق بن راهويه · The Book of Fasting

"We were with 'Ammar and a roast sheep was brought and he said: 'Eat.' One of the people turned away and said: 'I am fasting 'Ammar said: Whoever fasts on the day concerning which there is doubt, has disobeyed Abu Al-Qasim."' '

"The Messenger of Allah used to be keen to fast on Mondays and Thursday."