This is baseless. He was born on Monday and he was fasting that day. He never fasted 12 of rabee al awwal, neither did his companions!
Those who want to celebrate it may fast every Monday if they are truthful!

A friend of mine posted some hadith and said that the Prophet celebrated his own birthday. It is from sahih muslim vol2, book of Sawm, Hadith no. 1162. Is this authentic hadith?

Biography of the Prophet salla Allahu alahi wa sallam | December 27, 2015 7:28 pm
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The worshiper may not stay steadfast when doubts arise due to his ignorance. Sheikh please clarify that is there any hadith which states that end of Prophethood doesn’t mean end of Messenger-ship? Kindly also mention few Sahih ahadith which explicitly state that Messenger-ship has been terminated.
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Did Prophet Muhammad salla Allahu ‘alaihi wa sallam see Allah ‘azza wa jal?