Sunan an-Nasai Book 21, Hadith 2036

أَخْبَرَنَا إِسْحَاقُ بْنُ مَنْصُورٍ، قَالَ حَدَّثَنَا عَبْدُ الرَّحْمَنِ، عَنْ سُفْيَانَ، عَنْ أَبِي إِسْحَاقَ، عَنْ أَبِي الْخَلِيلِ، عَنْ عَلِيٍّ، قَالَ ‏:‏ سَمِعْتُ رَجُلاً، يَسْتَغْفِرُ لأَبَوَيْهِ وَهُمَا مُشْرِكَانِ فَقُلْتُ ‏:‏ أَتَسْتَغْفِرُ لَهُمَا وَهُمَا مُشْرِكَانِ فَقَالَ أَوَلَمْ يَسْتَغْفِرْ إِبْرَاهِيمُ لأَبِيهِ ‏.‏ فَأَتَيْتُ النَّبِيَّ صلى الله عليه وسلم فَذَكَرْتُ ذَلِكَ لَهُ فَنَزَلَتْ ‏{‏ وَمَا كَانَ اسْتِغْفَارُ إِبْرَاهِيمَ لأَبِيهِ إِلاَّ عَنْ مَوْعِدَةٍ وَعَدَهَا إِيَّاهُ ‏}‏ ‏.‏

It was narrated that 'Ali said:"I heard a man praying for forgiveness for his parents who were idolaters, and I said: 'Are you praying for forgiveness for them even though they are idolators?" He said: 'Didn't Ibrahim pray for forgiveness for his father?' I went to the Prophet and told him about that, then the following revealed: And Ibrahim's (Abraham) invoking (of Allah) for his father's forgiveness was only because of a promise he (Ibrahim) had made to him (his father)." (Daif)