The Prophet ﷺ said: If any Muslim dies and three rows of Muslims pray over him, it will assure him (of Paradise). When Malik considered those who accompanied a bier to be a few, he divided them into three rows in accordance with this tradition.
قَالَ رَسُولُ اللَّهِ ﷺ مَا مِنْ مُسْلِمٍ يَمُوتُ فَيُصَلِّي عَلَيْهِ ثَلاَثَةُ صُفُوفٍ مِنَ الْمُسْلِمِينَ إِلاَّ أَوْجَبَ قَالَ فَكَانَ مَالِكٌ إِذَا اسْتَقَلَّ أَهْلَ الْجَنَازَةِ جَزَّأَهُمْ ثَلاَثَةَ صُفُوفٍ لِلْحَدِيثِ
“If a funeral procession was brought and the number of people who followed it was considered to be small, they would be organized into three rows, then the funeral prayer would be offered.” He said: “The Messenger of Allah ﷺ said: ‘No three rows of Muslims offer the funeral prayer for one who has died, but he will be guaranteed (Paradise).’”
فَتَقَالَّ مَنْ تَبِعَهَا جَزَّأَهُمْ ثَلاَثَةَ صُفُوفٍ ثُمَّ صَلَّى عَلَيْهَا وَقَالَ إِنَّ رَسُولَ اللَّهِ ﷺ قَالَ مَا صَفَّ صُفُوفٌ ثَلاَثَةٌ مِنَ الْمُسْلِمِينَ عَلَى مَيِّتٍ إِلاَّ أَوْجَبَ
[Machine] The Messenger of Allah ﷺ said, "There is no Muslim who is prayed upon by three rows except that it is obligatory." He said, "And Malik used to divide them into three rows when he received the funeral procession."
قَالَ رَسُولُ اللهِ ﷺ «مَا مِنْ مُسْلِمٍ يُصَلِّي عَلَيْهِ ثَلَاثُ صُفُوفٍ إِلَّا أَوْجَبَ» قَالَ وَكَانَ مَالِكٌ إِذَا اسْتَقْبَلَ أَهْلَ الْجَنَازَةِ جَزَّأَهُمْ ثَلَاثَةَ صُفُوفٍ
"مَا مِنْ مُسْلِم يَمُوتُ فَيُصَلِّى عَليه ثَلاثَةُ صُفُوفٍ من المسلمين إِلَّا أَوْجَبَ".