Sayyar bin al-Ma'rur (may Allah have mercy on him) said: I heard Sayyiduna Umar bin al-Khattab (may Allah be pleased with him) say in a sermon: "O people! The Prophet (peace and blessings be upon him) built this mosque while we, the Muhajirun and the Ansar, were with him (peace and blessings be upon him). So when the crowd becomes heavy, let a man among you prostrate on the back of his brother."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجمعة / 5629