"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) (Using translation from Nasāʾī 2036)
سَمِعْتُ رَجُلًا يَسْتَغْفِرُ لِأَبَوَيْهِ وَهُمَا مُشْرِكَانِ فَقُلْتُ أَتَسْتَغْفِرُ لَهُمَا وَهُمَا مُشْرِكَانِ؟ فَقَالَ أَوَلَمْ يَسْتَغْفِرْ إِبْرَاهِيمُ لِأَبِيهِ فَأَتَيْتُ النَّبِيَّ ﷺ فَذَكَرْتُ ذَلِكَ لَهُ فَنَزَلَتْ {وَمَا كَانَ اسْتِغْفَارُ إِبْرَاهِيمَ لِأَبِيهِ إِلَّا عَنْ مَوْعِدَةٍ وَعَدَهَا إِيَّاهُ} [التوبة 114]