Musnad Abu Ya'la al-Mawsili 3:75

Ubayd Allah narrated to us, Yahya ibn Sa'id and Abd al-Rahman ibn Mahdi narrated to us, from Sufyan ibn Sa'id, from Abu Ishaq. Yahya said in his narration: Abu Ishaq narrated to us, from Abd Allah ibn al-Khalil, from Ali. And Abd al-Rahman said: 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 spoke to him about it. He said: "Did Ibrahim not seek forgiveness for his father?" I went to the Prophet, peace and blessings of Allah be upon him, and mentioned that to him. Then Allah revealed: "And Ibrahim’s seeking of forgiveness for his father was only because of a promise he had made to him." (At-Tawbah: 114)