What is the origin of the Imam's saying before prayer: "Align the shoulders and feet," and is it obligatory to join the feet together to close the gaps?

The saying "He who joins a row, Allah joins him; and he who severs a row, Allah severs him" has not been reported as directly attributed to the Prophet, peace and blessings be upon him. However, a similar meaning is found in a narration by Imam Ahmad and others: "Straighten your rows, for you are aligning with the rows of the angels. Align your shoulders, fill the gaps, be gentle with your brothers' hands, and do not leave any openings for Satan. He who joins a row, Allah joins him; and he who severs a row, Allah severs him, the Mighty and Majestic." Straightening the rows is considered desirable by the majority of scholars and obligatory by some. Joining foot to foot and not leaving a gap is a Sunnah.