Musnad Ahmad 2:136

Sulayman ibn Dawud Abu Dawud narrated to us, Salam—meaning Abu al-Ahwas—narrated to us, from Simak ibn Harb, from Sayyar ibn al-Ma'rur, who said: I heard Umar delivering a sermon, and he was saying: "The Messenger of Allah (peace and blessings of Allah be upon him) built this mosque, and we were with him: the Muhajirun and the Ansar. So, if the crowding becomes intense, let a man among you prostrate upon the back of his brother." And he saw some people praying in the road, so he said: "Pray in the mosque."