Musnad Ahmad 5:521

Waki’ narrated to us, from Sufyan—and [another chain] Abd al-Rahman narrated to us, he said: Sufyan narrated to us, from Abu Ishaq, from Abu al-Khalil, from Ali, who said: I heard 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 replied: "Did not Abraham seek forgiveness for his father while he was a polytheist?" He said: So I mentioned that to the Prophet, peace and blessings of Allah be upon him, and there was revealed: "It is not for the Prophet and those who have believed to ask forgiveness for the polytheists" [At-Tawbah: 113] until the end of the two verses. Abd al-Rahman said: Then Allah revealed: "And the request of forgiveness of Abraham for his father was only because of a promise he had made to him" [At-Tawbah: 114].