Hammad bin Salama حماد بن سلمة · The Book of Leading the Prayer (Al-Imamah)

"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."'