It is narrated from Sayyiduna Ali (may Allah be pleased with him) that I heard a man seeking forgiveness for his polytheist parents, so I said, "Are you seeking forgiveness for your polytheist parents?" He replied, "Did not Sayyiduna Ibrahim (peace be upon him) seek forgiveness for his polytheist father?" I mentioned this to the Prophet (peace and blessings be upon him), so this verse was revealed: «﴿مَاكَانَلِلنَّبِيِّوَالَّذِينَآمَنُواأَنْيَسْتَغْفِرُوالِلْمُشْرِكِينَ .....﴾ [التوبة : 113]» "It is not appropriate for the Prophet and the believers to seek forgiveness for the polytheists."
Hadith Referenceمسند احمد / مسند الخلفاء الراشدين / 1085