Musnad Ahmad 1099:14

Yunus and ‘Affan narrated to us, they said: Hammad ibn Salamah narrated to us, from ‘Ali ibn Zayd, from Abu al-Tufayl: That a man had a boy born to him during the time of the Messenger of Allah, may Allah bless him and grant him peace. He brought him to the Prophet, may Allah bless him and grant him peace, and he took hold of the skin of his forehead and prayed for blessing for him. He said: So a hair grew on his forehead in the shape of a bow. The boy grew up, and when the time of the Khawarij came, he loved them, so the hair fell from his forehead. His father took him, shackled him, and imprisoned him, fearing that he would join them. He said: So we entered upon him and admonished him, and we said to him among what we said: "Do you not see that the blessing of the supplication of the Messenger of Allah, may Allah bless him and grant him peace, has fallen from your forehead?" We did not cease with him until he turned back from their opinion, and Allah returned the hair to his forehead afterward, and he repented.