It is narrated from Sayyiduna Amr bin Tha'labah al-Juhani (may Allah be pleased with him); he said: I met the Messenger of Allah (peace and blessings be upon him) at al-Sayyalah and accepted Islam, and he (peace and blessings be upon him) passed his hand over my head. The narrator said: A hundred years passed over Amr bin Tha'labah, yet the place on his head where the hand of the Prophet (peace and blessings be upon him) had been never turned white.
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 14156