Celebrating birthdays has no source whatsoever in the pure shariah. In fact, it is an innovation, since the Messenger of Allah (sallallaahu ‘alaihi-wasallam) said,
“Whoever introduces anything into this matter of ours that does not belong to it shall have it rejected.”
This was recorded by al-Bukhari and Muslim. In a version recorded by Muslim and by al-Bukhari in definitive muallaq form.
"Whoever performs a deed which is not in accord with our affairs, that deed is rejected.”
It is well-known that the Prophet (sallallaahu ‘alaihi-wasallam) did not celebrate his birthday at all during his lifetime nor did he ever order it to be celebrated. Nor did he teach such to his Companions. Therefore, the rightly-guided caliphs and all of his Companions did not celebrate it. They are the most knowledgeable of the people concerning his sunnah and they are the most beloved to the Prophet (sallallaahu ‘alaihi-wasallam). They were also the most keen upon following whatever the Prophet (sallallaahu ‘alaihi-wasallam) brought. Therefore, if one is supposed to celebrate the Prophet (sallallaahu ‘alaihi-wasallam) birthday, this would have been made evident at their time. Similarly, not one of the scholars of the best of generations celebrated his birthday nor did they order it to be done.
Therefore, it is known from the above that such a celebration is not from the Law that Allah sent Muhammed (sallallaahu ‘alaihi-wasallam) with. We ask Allah and all Muslims to witness that if the Prophet (sallallaahu ‘alaihi-wasallam) had done so or ordered such to be done, or even if the Companions had done so, we would rush to do it and call others to do it. This is because, and all praises are due to Allah, we are the most keen in following his sunnah and respecting his commands and prohibitions. We ask Allah, for ourselves and for all our brethren Muslims, steadfastness upon the truth, avoiding everything that differs from Allah’s pure shariah. Verily, He is Generous and Noble.