that he said: "O Messenger of Allah ﷺ! If I have a son after you do you think I could name him your Kunyah?" He said: "Yes." So he said: "So that was permitted for me."
أَنَّهُ قَالَ يَا رَسُولَ اللَّهِ أَرَأَيْتَ إِنْ وُلِدَ لِي بَعْدَكَ أُسَمِّيهِ مُحَمَّدًا وَأُكْنِيهِ بِكُنْيَتِكَ قَالَ نَعَمْ قَالَ فَكَانَتْ رُخْصَةً لِي هَذَا حَدِيثٌ صَحِيحٌ
ʿAli ؓ said: O Messenger of Allah ﷺ, if I have a son after you are gone, I will call him by your name and give him your kunyah. He said: ʿYes.ʿ That was a concession from the Messenger of Allah ﷺ to ʿAli.
عَلِيٌّ يَا رَسُولَ اللهِ أَرَأَيْتَ إِنْ وُلِدَ لِي بَعْدَكَ وَلَدٌ أُسَمِّيهِ بِاسْمِكَ وَأُكَنِّيهِ بِكُنْيَتِكَ؟ قَالَ نَعَمْ فَكَانَتْ رُخْصَةً مِنْ رَسُولِ اللهِ ﷺ لِعَلِيٍّ
ʿAli ؓ said: O Messenger of Allah ﷺ, if I have a son after you are gone, I will call him by your name and give him your kunyah. He said: ʿYes.ʿ That was a concession from the Messenger of Allah ﷺ to ʿAli. (Using translation from Aḥmad 730)
سَمِعْتُ أَبِي يَقُولُ قُلْتُ يَا رَسُولَ اللَّهِ أَرَأَيْتَ أَنْ وُلِدَ لِي بَعْدَكَ وَلَدٌ أُسَمِّيهِ بِاسْمِكَ وَأُكَنِّيهِ بِكُنْيَتِكَ؟ قَالَ «نَعَمْ» قَالَ عَلِيٌّ «فَكَانَتْ هَذِهِ رُخْصَةً لِي»
"عَنْ عليٍّ قَالَ: قلتُ: يا رَسُولَ الله أرأَيتَ إِنْ وُلِدَ لِى بَعْدَكَ وَلَدٌ أُسَمِّيه باسمِكَ، وَأُكنِّيهِ بِكُنيَتِكَ؟ فَقَالَ: نَعَمْ فكانْت رخصةً من رَسُولِ الله ﷺ لَعِلىٍّ".