Musnad Ahmad 27:1274

Harun ibn Ma'ruf narrated to us, Abdullah ibn Wahb narrated to us, from Mu'awiyah ibn Salih, from Abu al-Zahiriyah, from Kathir ibn Murrah, from Abdullah ibn Umar, that the Messenger of Allah (peace and blessings of Allah be upon him) said: "Establish the rows, for you only align yourselves in the rows of the angels. Align the shoulders, fill the gaps, be gentle in the hands of your brothers, and do not leave gaps for the devil. Whoever connects a row, Allah, Blessed and Exalted is He, connects him; and whoever severs a row, Allah, Blessed and Exalted is He, severs him."