Shaykh ‘Abd al-‘Azeez ibn Baaz (may Allaah have mercy on him) said:

If celebrating the Prophet’s birthday was prescribed, then the Prophet (peace and blessings of Allaah be upon him) would have told his ummah of that, because he is the most sincere of people and there is no Prophet after him who could explain anything he did not speak about. He (peace and blessings of Allaah be upon him) is the Seal of the Prophets and he explained to the people what he had to explain of the truth, such as loving him and following his sharee’ah, sending blessings and salaams upon him and other rights of his that are explained in the Qur’aan and Sunnah. He did not tell his ummah that celebrating the day of his birth was something prescribed so that they would do that. He (peace and blessings of Allaah be upon him) did not do that during his lifetime and his companions (peace and blessings of Allaah be upon him) who were the dearest of people to him and the most knowledgeable of his rights did not celebrate that day, neither did the Rightly-Guided Caliphs or any others. Then those who followed them in truth of the best three generations did not celebrate this day either.

Do you think that all these people were ignorant of his rights or fell short with regard to them, until the later generations came and made up for this shortfall and made the truth complete? No, by Allaah. No wise man who understands the nature of the Sahaabah and how they followed the truth would say this. If you understand that the celebration of the Prophet’s birthday was unknown at the time of the Prophet (peace and blessings of Allaah be upon him) and the time of his companions and the time of their earliest followers, you will realize that it is an innovation that has been introduced into the faith, and it is not permissible to do it, approve of it or advocate it, rather we must denounce it and warn people against it.

[Majmoo’ Fataawa al-Shaykh Ibn Baaz, 6/318, 319]