Anas said

The iqama* had been uttered and Allah's Messenger ﷺ faced us and said, “Make your rows straight and stand close together, for l can see you behind my back.” * A repetition of the words of the call to prayer after the worshippers have taken their places with the addition of a statement that the time of prayer has come. Bukhari transmitted it. In the version given by Bukhari and Muslim he said, “Make the rows perfect, for I can see you behind my back."