Reference: Fataawa al-Lajnah ad-Daa.imah lil-Buhooth al-‘Ilmiyyah wal-Iftaa. – Volume 3, Page 30, Question 13, Fatwa No. 5005
Question: Is gathering [the people] on the 12th day of Rabee’ al-Awwal in remembrance of the birth of the Prophet ﷺ permissible?
Response: Celebrating the birthday of the Messenger ﷺ is an innovation; this is because he ﷺ did not do this himself and nor did he order this to be done, and nor did any of his companions (radhi-yAllaahu ‘anhum) do this, and they were the most adherent of people in honouring the Messenger ﷺ and following his Sunnah. And all good is in following his guidance, and he ﷺ said:
«Whoever introduces anything into this matter of ours (i.e. Islaam) that which is not from it, will have it rejected», [Saheeh al-Bukhaaree/2697 and Saheeh Muslim/1718].
And with Allaah lies all success and may Allaah send prayers and salutations upon our Prophet ﷺ and his family and his companions.