Tafsir of Ash-Shu`ara' 26:86

"And forgive my father," Ibn Abbas said, as recorded from him by Ibn Abi Hatim, meaning: "Bestow upon him a favor by way of a repentance through which he becomes deserving of Your forgiveness." Its essence is: "Grant him success in attaining faith," as is implied by his providing the reason for it in his saying, "for he was of the astray."

This is clear if this supplication occurred before his father's death. If it occurred after his death, then the supplication for forgiveness is understood according to its apparent meaning, and it was permissible to supplicate for it on behalf of a polytheist. God Almighty does not forgive that any partner be associated with Him, but he [Abraham], peace be upon him, had not yet received revelation concerning that at that time, and reason does not dictate the impossibility [of such a supplication prior to the prohibition]. In Sharh Muslim by an-Nawawi, it is stated that the fact that He, the Almighty, does not forgive polytheism is specific to this Ummah, and before them, it may have been forgiven—though this is a matter of debate.

It has also been said: It is because he [the father] was concealing his faith out of taqiyya (precautionary dissimulation) from Nimrod, and for that reason, he [Abraham] promised him [to pray for] forgiveness. But when it became clear to him—by way of revelation or at the time of [the father's] death—that he was an enemy to faith, he disassociated himself from him. According to this view, his saying "of the astray" is based on what appeared to others regarding his state, or it means: "of the astray" in concealing his faith and not acknowledging it with his tongue out of taqiyya from Nimrod. Discussion on this topic is lengthy, and some of it has preceded, so reflect upon it.