Reference: Fataawa al-Lajnah ad-Daa.imah lil-Buhooth al-‘Ilmiyyah wal-Iftaa. – Volume 3, Page 6, Question 1, Fatwa No. 7136
Question: Celebrating the birthday of the Prophet (sal-Allaahu ‘alayhi wa sallam), gathering people together and talking about the life of the Messenger and performing the salaah and finding out about the attributes of the Messenger and openly sending salutations to him.
Response: Establishing celebrating the occasion of his (sal-Allaahu ‘alayhi wa sallam) birthday is not permissible, due to it being an innovation, [something] which the Messenger of Allaah (sal-Allaahu ‘alayhi wa sallam) and nor his rightly-guided Khulafaa. did, and nor other than them from the scholars of the chosen first three generations.
And with Allaah lies all success and may Allaah send prayers and salutations upon our Prophet (sal-Allaahu ‘alayhi wa sallam) and his family and his companions.