ANSWER:

He, salla Allahu alaihi wa sallam used to fast every Monday since he was born on a monday.  The Prophet salla Allahu alaihi wa sallam also justified fasting on Monday because it was a day that he was revealed to in it. 

I don’t give references here due to lack of time.

Refer to  islamqa.info for detailed answers with references. 

Is there any reference that Prophet Muhammad salallahu walahi wa sallam fasted on his birthday?

Biography of the Prophet salla Allahu alahi wa sallam, Fasting | August 2, 2022 1:30 pm
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Are we supposed to fast the first 10 days of Muharram and the 9th and 10th day of Muharram, is it a sunnah of prophet, pls clarify sheikh
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As Prophet Muhammad salallahu alahi wa sallam fasted on his birthday so should we also first on our birthday or will it be a form of celebrating birthdays?