Musannaf Ibn Abi Shayba 38:148

Aswad ibn Amir narrated to us, he said: Hammad ibn Salamah narrated to us, from Ali ibn Zayd, from Abu al-Tufayl, that a boy was born to a man during the time of the Prophet, may God bless him and grant him peace. He prayed for him, took hold of the skin of his forehead, moved it like this, pressed his forehead, and prayed for blessing for him. He said: A hair grew on his forehead as if it were the coarse hair of a horse. 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 and shackled him for fear that he would join them. He said: We entered upon him and exhorted him, and we said to him, among what we said: "Do you not see that the blessing of the supplication of the Messenger of God, may God bless him and grant him peace, has fallen from your forehead?" We did not cease with him until he turned back from their opinion. He said: So God returned the hair to his forehead afterward, and he repented and reformed.