Musnad Ahmad 30:1242

Abu Asim narrated to us, Muhammad ibn Rifa'ah narrated to us, from Suhayl ibn Abi Salih, from his father, from Abu Hurayrah, that the Messenger of Allah (peace and blessings of Allah be upon him) used to fast most often on Mondays and Thursdays. It was said to him [about that], so he said: "Indeed, deeds are presented every Monday and Thursday—or: every Monday and Thursday—so Allah, the Mighty and Majestic, forgives every Muslim—or: every believer—except those who have forsaken one another. He says: 'Defer these two.'"