Sayyiduna Abdullah bin Umar (may Allah be pleased with him) narrates that the Messenger of Allah (peace and blessings be upon him) said: Straighten your rows, (keep in mind that) you have to form rows like the rows of angels, keep your shoulders aligned, fill the gaps, be gentle in the hands of your brothers, and do not leave any empty space for Satan. (Remember that) whoever joins the row, Allah will join him (with His grace and mercy), and whoever cuts the row, Allah will cut him (off from His mercy).