The Messenger of Allah ﷺ was asked about fasting on Mondays. He replied, "On that day I was born, and on that day revelation was sent down to me."
أن رسول الله ﷺ سئل عن صوم الاثنين؟ فقال: فيه ولدت، وفيه أنزل علي.
Muslim narrated it in the Book of Fasting (1162:198) via Mahdi bin Maymun, from Ghaylan (that is, Ibn Jareer Mughuli), from Abdullah bin Mabad Zimmani, from Abu Qatada, and he related it.He also narrated it (1162:197) via Shubah, from Ghaylan bin Jareer, with the same account, similarly.Then Muslim said, "In this hadith, from the narration of Shubah, he said, 'He was asked about fasting Monday and Thursday.' We omitted the mention of Thursday because we consider it a mistake."