Ali bin Abu Talib reported,
Ubayy reported, "I heard a man seeking forgiveness for his parents, who were both polytheists, so I said to him, 'Are you seeking forgiveness for them when they are polytheists?'He replied, 'Did Ibrahim not seek forgiveness for his father?'So I went to the Prophet ﷺ and mentioned this to him, and the verse was revealed, 'And Ibrahim's request for forgiveness for his father was only because of a promise he had made to him' [Tawbah 9:114]."
Azami 12522
Hasanعن علي قال
سمعت رجلا يستغفر لأبويه، وهما مشركان، فقلت: أتستغفر لهما وهما مشركان. فقال: أو لم يستغفر إبراهيم لأبيه، فأتيت النبي ﷺ، فذكرت ذلك له فنزلت: {وما كان استغفار إبراهيم لأبيه إلا عن موعدة وعدها إياه}
Tirmidhi (3101), Nasai (2036), Ahmad (1085), and Hakim (2/335) all narrated it, each through the hadith of Sufyan, from Abu Ishaq, from Abu Khalil, from Ali bin Abu Talib, who related it.Tirmidhi said, "A hasan hadith." Hakim said, "Its isnad is sahih."I say, Abu Khalil is Abdullah bin Khalil, or Ibn Abu Khalil, the Hadrami, the Kufan. Ibn Saad said, "He narrated few hadiths." Ibn Hibban deemed him reliable, and a number of narrators transmitted from him, so his hadith is considered hasan when it serves as a corroborating narration. But when he narrates alone, it must be examined closely.As for the phrase "he had promised him" [Tawbah 9:114], it means his father. This is because Ibrahim 
had promised his father that he would seek forgiveness for him, hoping he would embrace Islam, and his asking forgiveness took place while his father was still a polytheist. "But when it became clear to him that he was an enemy of Allah, he disowned him. Indeed, Ibrahim was compassionate and forbearing" [Tawbah 9:114], and that disavowal came after his father died in disbelief. It has indeed come in the Sahih: