It was narrated that Al-Bara bin Azib said: "The Messenger of Allah (ﷺ) used to go between the rows from one side to another, patting our shoulders and chests and saying: 'Do not make your rows ragged or your hearts will be filled with enmity toward one another.' And he used to say: 'Allah and His angels send Salah upon the front rows."'
Source: sunnah.com
Other translations
Arabic
أَخْبَرَنَا قُتَيْبَةُ بْنُ سَعِيدٍ، قَالَ حَدَّثَنَا أَبُو الأَحْوَصِ، عَنْ مَنْصُورٍ، عَنْ طَلْحَةَ بْنِ مُصَرِّفٍ، عَنْ عَبْدِ الرَّحْمَنِ بْنِ عَوْسَجَةَ، عَنِ الْبَرَاءِ بْنِ عَازِبٍ، قَالَ كَانَ رَسُولُ اللَّهِ صلى الله عليه وسلم يَتَخَلَّلُ الصُّفُوفَ مِنْ نَاحِيَةٍ إِلَى نَاحِيَةٍ يَمْسَحُ مَنَاكِبَنَا وَصُدُورَنَا وَيَقُولُ " لاَ تَخْتَلِفُوا فَتَخْتَلِفَ قُلُوبُكُمْ " . وَكَانَ يَقُولُ " إِنَّ اللَّهَ وَمَلاَئِكَتَهُ يُصَلُّونَ عَلَى الصُّفُوفِ الْمُتَقَدِّمَةِ " .
sunnah.com