(21) The Book of Funerals
كتاب الجنائز
(87) Chapter: It Is Recommended To Make The Grave Wide
(87) باب مَا يُسْتَحَبُّ مِنْ تَوْسِيعِ الْقَبْرِ ‏‏
Sunan an-Nasa'i 2011 
It was narrated from Sa'eed bin Hisham bin 'Amir that his father said:
"On the day of Uhud some of the people among the Muslims were killed, and people were wounded. The Messenger of Allah said: 'Dig graves and make them wide, and bury two or three in a grave, and put the one who knew more Qur'an in first."'
أَخْبَرَنَا مُحَمَّدُ بْنُ مَعْمَرٍ، قَالَ حَدَّثَنَا وَهْبُ بْنُ جَرِيرٍ، قَالَ حَدَّثَنَا أَبِي قَالَ، سَمِعْتُ حُمَيْدَ بْنَ هِلاَلٍ، عَنْ سَعْدِ بْنِ هِشَامِ بْنِ عَامِرٍ، عَنْ أَبِيهِ، قَالَ ‏:‏ لَمَّا كَانَ يَوْمُ أُحُدٍ أُصِيبَ مِنَ الْمُسْلِمِينَ، وَأَصَابَ النَّاسَ جِرَاحَاتٌ فَقَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم ‏:‏ ‏ "‏ احْفِرُوا وَأَوْسِعُوا، وَادْفِنُوا الاِثْنَيْنِ وَالثَّلاَثَةَ فِي الْقَبْرِ، وَقَدِّمُوا أَكْثَرَهُمْ قُرْآنًا ‏"‏ ‏.‏
Grade :  Sahih (Darussalam)
Reference : Sunan an-Nasa'i 2011
In-book reference : Book 21, Hadith 194
English translation : Vol. 3, Book 21, Hadith 2013