The Prophet (ﷺ) said: Have the imam in the centre and close up the gaps.
Sunan Abi Dawud Book 2, Hadith 291
← Previous Hadith Abu Sa’Id al-Khudri said; The Messenger of Allah (ﷺ) saw a tendency among his companions to go to the back. He said to them; come forward and follow my lead, and let those who come after you follow your lead people will continue to keep to the back till Allah would put them at the back.

Sunan Abi Dawud Book 2, Hadith 290 Next Hadith → The Messenger of Allah (ﷺ) saw a man praying alone behind the row. He ordered him to repeat. Sulayman ibn Harb said: The prayer.

Sunan Abi Dawud Book 2, Hadith 292