"ذلك يوم ولدت فيه، ويوم بعثت أو أنزل علي فيه"
Abu Qatadah (May Allah be pleased with him) reported: 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] .