Sahih Ibn Khuzaymah

Sayyiduna Bara bin Azib (may Allah be pleased with him) narrates that when we would stand for prayer, the Messenger of Allah (peace and blessings be upon him) would come to us and straighten our shoulders and chests with his blessed hand and say: "Your chests should not be uneven (ahead or behind), otherwise your hearts will also become different. Indeed, Allah the Exalted sends His mercy upon those in the first row, and His angels pray for mercy and forgiveness for them." And the Messenger of Allah (peace and blessings be upon him) said: "Adorn the Qur'an (with your voices)." Janab Abdur Rahman bin Awsajah says that I had forgotten these words: "Adorn the Qur'an with your voices," until Dahhak bin Muzahim reminded me of these words.

Sayyiduna Bara’ bin Azib (may Allah be pleased with him) narrates that the Messenger of Allah (peace and blessings be upon him) used to come to us (at the time of arranging rows). He would straighten our shoulders and chests with his blessed hand and say: “Your rows should not be crooked, otherwise your hearts will also become crooked. Indeed, Allah the Exalted sends His mercy upon the first row or the first rows, and His angels pray for their forgiveness.”