Short answer
No. There is no sunnah of fasting on your birthday. The Prophet ﷺ fasted on Mondays and gave as one reason that he was born on that day, but that was a weekly fast, not a yearly one on his date of birth. You may fast on your birthday as an ordinary voluntary fast, without believing it carries a special reward.
A voluntary fast on any day except the forbidden days is permissible and rewarded. Treating one's birthday as a day with a special sunnah fast has no basis, and teaching it as an established practice of the religion should be avoided.
The four schools agree on this.
Explanation
When the Prophet ﷺ was asked about fasting on Mondays, he said: on it I was born, and on it revelation came to me. Some people take from this that fasting on one's own birthday is sunnah. But look at what he did: he fasted every Monday, a weekly day, and did not single out his date of birth each year. No Companion is reported to have fasted on his birthday as a practice.
What this means for you:
- If you want to fast on your birthday as a voluntary fast, you may, and it is rewarded like any nafl fast.
- Do not believe it has a special reward of its own, or teach it to others as a sunnah.
- If your birthday falls on a Monday, a Thursday or one of the white days, intend that sunnah and you have both.
- Do not fast if it falls on Eid or the days of tashreeq. See fasting on Eid.
A better habit is the one the Prophet ﷺ actually kept: fasting on Mondays, every week. See fasting Mondays and Thursdays and the full list of sunnah fasts. For whether marking a birthday at all is allowed, see are birthdays haram.
The four schools agree that a voluntary fast is allowed on any permitted day, and none of them lists the birthday as a sunnah fast.
The evidence
Sahih Muslim 1162
Sahih... فيه ولدت وفيه أنزل علي ...
"On it I was born, and on it revelation was sent down to me."
The Prophet ﷺ fasted Mondays every week, not his birthday once a year.
Where the ruling is recorded
- Radd al-Muhtar (Ibn Abidin), Kitab al-Sawm, opening section on the kinds of fasting. The recommended fasts are those with a basis in the sunnah, such as Mondays and Thursdays.
Checked by our team. For a more accurate answer to your own situation, consult a scholar. Quran translation by Sahih International. Hadith Arabic from the standard editions of the six books and the Muwatta, with gradings as recorded there; the English is our own rendering. Spotted a mistake? Tell us.