Sayyar bin Ma'rur narrates: I heard Sayyiduna Umar (may Allah be pleased with him) delivering a sermon, saying: "The Prophet (peace and blessings be upon him) built this mosque, and we, the Muhajirun and the Ansar, were with him. When the crowd becomes heavy, let one of you prostrate on the back of his brother." He saw some people praying in the road, and he said: "Pray in the mosque."
Ahmad recorded it; the narrator Sayyar is majhul (unknown), and it is said about him (that is, about his father's name) that it is Maghrur, with both the dotted and the undotted letter.
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الصلاة / 1954