Hadith 1255
Abu Qatadah (May Allah be pleased with him) reported:
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].
In plain words
The Prophet linked fasting on Mondays to meaningful moments in his own life. It shows that this day held a special place for him, which is why he marked it with worship.
Reference : Riyad as-Salihin 1255 In-book reference : Book 8, Hadith 265