Sunan an-Nasa'i, Hadith 816 ⓘ
It was narrated that Jabir bin Samurah said: "The Messenger of Allah came out to us and said:'Will you not form rows as the angels form rows before their Lord? They said: 'How do the angels form rows before their lord? He said: 'They complete the first row and fill the gaps in the rows."'
أَخْبَرَنَا قُتَيْبَةُ، قَالَ حَدَّثَنَا الْفُضَيْلُ بْنُ عِيَاضٍ، عَنِ الأَعْمَشِ، عَنِ الْمُسَيَّبِ بْنِ رَافِعٍ، عَنْ تَمِيمِ بْنِ طَرَفَةَ، عَنْ جَابِرِ بْنِ سَمُرَةَ، قَالَ خَرَجَ إِلَيْنَا رَسُولُ اللَّهِ صلى الله عليه وسلم فَقَالَ " أَلاَ تَصُفُّونَ كَمَا تَصُفُّ الْمَلاَئِكَةُ عِنْدَ رَبِّهِمْ " . قَالُوا وَكَيْفَ تَصُفُّ الْمَلاَئِكَةُ عِنْدَ رَبِّهِمْ قَالَ " يُتِمُّونَ الصَّفَّ الأَوَّلَ ثُمَّ يَتَرَاصُّونَ فِي الصَّفِّ " .
also referenced under: Book 10, Hadith 40 (In-Book) & Volume 1, Book 10, Hadith 817 (DarusSalam)