Musnad Ahmad 217
It was narrated that Sayyar bin al-Ma'roor said:
I heard 'Umar giving a khutbah, and he said: The Messenger of Allah (ﷺ) built this mosque and we, the Muhajireen and Ansar, were with him. If it gets too crowded, then let one of you prostrate on his brother's back. And he saw people (on one occasion) praying in the street and said: Pray in the mosque,
حَدَّثَنَا سُلَيْمَانُ بْنُ دَاوُدَ أَبُو دَاوُدَ، حَدَّثَنَا سَلَّامٌ يَعْنِي أَبَا الْأَحْوَصِ، عَنْ سِمَاكِ بْنِ حَرْبٍ، عَنْ سَيَّارِ بْنِ الْمَعْرُورِ، قَالَ سَمِعْتُ عُمَرَ، رَضِيَ اللَّهُ عَنْهُ يَخْطُبُ وَهُوَ يَقُولُ إِنَّ رَسُولَ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ بَنَى هَذَا الْمَسْجِدَ وَنَحْنُ مَعَهُ الْمُهَاجِرُونَ وَالْأَنْصَارُ فَإِذَا اشْتَدَّ الزِّحَامُ فَلْيَسْجُدْ الرَّجُلُ مِنْكُمْ عَلَى ظَهْرِ أَخِيهِ وَرَأَى قَوْمًا يُصَلُّونَ فِي الطَّرِيقِ فَقَالَ صَلُّوا فِي الْمَسْجِدِ ‏.‏