Sayyar bin Ma’rur says: I heard Sayyiduna ‘Umar (may Allah be pleased with him) delivering a sermon, and he was saying: Indeed, the Messenger of Allah (peace and blessings be upon him) built this mosque, while we, the Muhajirun and Ansar, were with him (peace and blessings be upon him). So when the crowd becomes large, a person should prostrate on the back of his brother. Then Sayyiduna ‘Umar (may Allah be pleased with him) saw some people praying on the road and said to them: Pray in the mosque.