Compiled by Imam Yahya ibn Sharaf al-Nawawi
عن أبي قتادة رضي الله عنه أن رسول الله صلى الله عليه وسلم: سئل عن صوم يوم الاثنين فقال: "ذلك يوم ولدت فيه، ويوم بعثت أو أنزل علي فيه" ((رواه مسلم)).
The Messenger of Allah (ﷺ) was asked about fasting on Mondays. He said, "That is the day on which I was born and the day on which I received Revelation."[Muslim].