The Prophet ﷺ informed his companions about the death of An-Najashi and then he went ahead (to lead the prayer) and the people lined up behind him in rows and he said four Takbir.  

نَعَى النَّبِيُّ ﷺ إِلَى أَصْحَابِهِ النَّجَاشِيَّ ثُمَّ تَقَدَّمَ فَصَفُّوا خَلْفَهُ فَكَبَّرَ أَرْبَعًا  

Ash Shuʿbi said, "I was informed by a man who had seen the Prophet ﷺ going to a grave that was separate from the other graves and he aligned the people in rows and said four Takbir." I said, "O Abu ʿAmr! who narrated (that) to you"? He said, "Ibn ʿAbbas. "  

أَخْبَرَنِي مَنْ شَهِدَ النَّبِيَّ ﷺ أَنَّهُ أَتَى عَلَى قَبْرٍ مَنْبُوذٍ فَصَفَّهُمْ وَكَبَّرَ أَرْبَعًا قُلْتُ مَنْ حَدَّثَكَ قَالَ ابْنُ عَبَّاسٍ ؓ  

The Prophet ﷺ said, "Today a pious man from Ethiopia (i.e. An Najashi) has expired, come on to offer the funeral prayer." (Jabir said): We lined up in rows and after that the Prophet ﷺ led the prayer and we were in rows.  

قَالَ النَّبِيُّ ﷺ قَدْ تُوُفِّيَ الْيَوْمَ رَجُلٌ صَالِحٌ مِنَ الْحَبَشِ فَهَلُمَّ فَصَلُّوا عَلَيْهِ قَالَ فَصَفَفْنَا فَصَلَّى النَّبِيُّ ﷺ عَلَيْهِ وَنَحْنُ صُفُوفٌ