Al-Imam Abu Hanifah, may Allah raise his rank, said (in his book Al-Fiqh Al-Akbar):
مَا مَاتَا كَافِرَيْن
which means: “The Parents of the Messenger of Allah did not die as blasphemers.”
As for the narration in which it says, the Prophet ﷺ said to a man:
إِنَّ أَبِيْ وَأَبَاكَ فِي النَّار
which means: “My father and your father in Hellfire”. This is a defective and unreliable hadith, even if Al-Imam Muslim narrated it. In the book of Al-Imam Muslim (Sahih Muslim) there are Hadiths which some of scholars Hadith mentioned as weak and unreliable and this Hadith is among them.
[This narration was judged as weak by Al-Hafidh As-Suyutiyy.]
As for the hadith in which it is mentioned: “the Messenger of Allah spent a lot of time at the grave of his mother and cried and then it was said to him “O Messenger of Allah we saw that you spent a long time at the grave of your mother and cried (they were asking like “why?”)” to which he responded “I asked the permission of my Lord to visit her and He granted me that and I requested to make istighfar for her (to seek forgiveness on her behalf) and He did not grant me that””.
This hadith is interpreted (a befitting meaning is given to it) to mean that: Allah prevented the Prophet ﷺ for making Istighfar for her so that the situation is not unclear for those people (i.e. the Companions) whose fathers and mothers died as idol worshippers to then seek forgiveness for their non-Muslim parents; and it was not because the mother of the Prophet ﷺ was a blasphemer. And this is the way to respond against those who took by the apparent meaning of that Hadith and said “the mother of the Prophet ﷺ was a blasphemer and that is why the Prophet ﷺ was not given permission to make Istighfar for her”.
The proof that (Lady Aminah bint Wahb) the mother of the Prophet ﷺ was a believers is that when she gave birth to him ﷺ a light shun which enabled her to see the castles of Ash-Sham and there is a vast distance between (where she was:) Makkah and Ash-Sham. She saw the castles of Bus-ra and Busra is among the old cities of Ash-Sham and is considered to be among the land of Haw-ran which lies next to Jordan. And so, the mother of the Prophet ﷺ saw the castles of Busra by this light that emitted from her when she gave birth to the Prophet ﷺ.
This hadith is confirmed and reliable and was narrated by Al-Hafidh ibn Hajar (Al-^Asqalaniyy) in his book Al-‘A-ma-liyy and he judged it to be Hasan.
The beholding of (Lady) Aminah radiyallahu ^anha of the castles of Busra is considered as karamah for her as this was something that was extraordinary (and the karamat only occur for the awliya`, so she was not only a Muslim but a waliyyah).
When Al-Imam Muslim authored his book Sahih Muslim he presented it to some of the Huffadh (plural of Hafidh) and they confirmed all of it except for four Hadiths. He himself said this in the beginning of his book but he did not mention which four those were. And Al-Bukhariyy classed two Hadith from his book as being weak. This was also said by Al-Hafidh ibn Hajar.
And even if, it was the case that the parents of the Prophet ﷺ were not Muslims then they were from among the People of Fat-rah – the People of Fat-rah were those people whom the call of previous the Prophets did not reach them and they are not punished in the Hereafter i.e. they are safe in the Hereafter. Allah ta^ala revealed in the Qur’an in Surah Al-Isra’ verse 15:
وَمَا كُنَّا مُعَذِّبِينَ حَتَّىٰ نَبْعَثَ رَسُولًا
which means “The one who did not hear the call of a Prophet will not be punished”