Ibn Shihab narrates: Sahl bin Sa'd (may Allah be pleased with him) said - and he had seen the Prophet (peace and blessings be upon him) and heard from him - that he was fifteen years old on the day the Messenger of Allah (peace and blessings be upon him) passed away.
Hadith Referenceالمعجم الكبير للطبراني / باب السين / 5633