Riyad as-Salihin, Book 9, Hadith 1255 (In-Book) ⓘ
Abu Qatadah (May Allah be pleased with him) reported: The Messenger of Allah 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].
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)).