Musnad Abu Ya'la al-Mawsili 3:359

Abu Khaythama narrated to us, Waki' narrated to us, from Sufyan, from Abu Ishaq, from Abu al-Khalil, from Ali, who said: I saw a man seeking forgiveness for his parents while they were polytheists. So I said, "Do you seek forgiveness for your parents while they are polytheists?" He said, "Did Ibrahim not seek forgiveness for his father while he was a polytheist?" He said: So I mentioned that to the Prophet (peace be upon him), and there was revealed: "It is not for the Prophet and those who have believed to ask forgiveness for the polytheists," until the end of the two verses.