Ata, the freed slave of Sayyiduna al-Sa'ib bin Yazid (may Allah be pleased with him), narrates: The middle of Sayyiduna al-Sa'ib's (may Allah be pleased with him) head was black, while the rest of his head and his beard were white. I said to him: My master, by Allah! I have never seen a head like this head of yours; this is white and this is black. He said: My son, shall I not tell you (the reason for it)? I said: Of course. He said: I was playing with some children when the Messenger of Allah (peace and blessings be upon him) passed by me. I came before him (peace and blessings be upon him) and greeted him with salam, and he (peace and blessings be upon him) said: "And upon you be peace. Who are you?" I said: I am al-Sa'ib bin Yazid, the brother of al-Namir bin Qasit. So the Messenger of Allah (peace and blessings be upon him) passed his hand over my head and said: "May Allah bless you." He said: By Allah! It (this part) will never turn white; or he said: It will always remain like this.
Hadith Referenceالمعجم الكبير للطبراني / باب السين / 6663