It was narrated that ‘Ali (رضي الله عنه) said: When al-Hasan was born, I named him Hamzah. When al-Husain was born, I named him after his paternal uncle Ja`far. Then the Messenger of Allah (ﷺ) called me and said: `I have been commanded to change the names of these two.` I said: Allah and His Messenger know best. And he named them Hasan and Husain. (رضي الله عنهما)
Source: sunnah.com
Other translations
Arabic
حَدَّثَنَا زَكَرِيَّا بْنُ عَدِيٍّ، أَنْبَأَنَا عُبَيْدُ اللَّهِ بْنُ عَمْرٍو، عَنْ عَبْدِ اللَّهِ بْنِ مُحَمَّدِ بْنِ عَقِيلٍ، عَنْ مُحَمَّدِ بْنِ عَلِيٍّ، عَنْ عَلِيٍّ، رَضِيَ اللَّهُ عَنْهُ قَالَ لَمَّا وُلِدَ الْحَسَنُ سَمَّاهُ حَمْزَةَ فَلَمَّا وُلِدَ الْحُسَيْنُ سَمَّاهُ بِعَمِّهِ جَعْفَرٍ قَالَ فَدَعَانِي رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ فَقَالَ إِنِّي أُمِرْتُ أَنْ أُغَيِّرَ اسْمَ هَذَيْنِ فَقُلْتُ اللَّهُ وَرَسُولُهُ أَعْلَمُ فَسَمَّاهُمَا حَسَنًا وَحُسَيْنًا.
sunnah.com