It was narrated from Abul-Khaleel that ‘Ali (رضي الله عنه) said: I heard a man asking for forgiveness for his parents and they were mushrikeen. I said: Are you asking for forgiveness for your parents when they are mushrikeen? He said: Didn`t Ibraheem (عليه السلام) ask for forgiveness for his father when he was a mushrik? I mentioned that to the Prophet (ﷺ) and these verses were revealed: “It is not (proper) for the Prophet (ﷺ) and those who believe to ask Allah`s forgiveness for the Mushrikeen” . ‘Abdur-Rahman said: And Allah revealed . “And Ibraheem`s (Abraham) invoking (of Allah) for his father`s forgiveness was only because of a promise he had made to him (his father)`.
Source: sunnah.com
Other translations
Arabic
حَدَّثَنَا وَكِيعٌ، عَنْ سُفْيَانَ، ح وَحَدَّثَنَا عَبْدُ الرَّحْمَنِ، قَالَ حَدَّثَنَا سُفْيَانُ، عَنْ أَبِي إِسْحَاقَ، عَنْ أَبِي الْخَلِيلِ، عَنْ عَلِيٍّ، رَضِيَ اللَّهُ عَنْهُ قَالَ سَمِعْتُ رَجُلًا، يَسْتَغْفِرُ لِأَبَوَيْهِ وَهُمَا مُشْرِكَانِ فَقُلْتُ تَسْتَغْفِرُ لِأَبَوَيْكَ وَهُمَا مُشْرِكَانِ فَقَالَ أَلَيْسَ قَدْ اسْتَغْفَرَ إِبْرَاهِيمُ لِأَبِيهِ وَهُوَ مُشْرِكٌ قَالَ فَذَكَرْتُ ذَلِكَ لِلنَّبِيِّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ فَنَزَلَتْ {مَا كَانَ لِلنَّبِيِّ وَالَّذِينَ آمَنُوا أَنْ يَسْتَغْفِرُوا لِلْمُشْرِكِينَ} إِلَى آخِرِ الْآيَتَيْنِ قَالَ عَبْدُ الرَّحْمَنِ فَأَنْزَلَ اللَّهُ {وَمَا كَانَ اسْتِغْفَارُ إِبْرَاهِيمَ لِأَبِيهِ إِلَّا عَنْ مَوْعِدَةٍ وَعَدَهَا إِيَّاهُ}.
sunnah.com