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].

عن أبي قتادة رضي الله عنه أن رسول الله ﷺ: سئل عن صوم يوم الاثنين فقال:
"ذلك يوم ولدت فيه، ويوم بعثت أو أنزل علي فيه" ((رواه مسلم)).