Nafi' (may Allah have mercy on him) narrates from Sayyiduna Ibn Umar (may Allah be pleased with them both) that the Messenger of Allah (peace and blessings be upon him) recited Surah al-Najm, then prostrated and prolonged the prostration. The people were very many, so they were prostrating on one another's backs.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجمعة / 5628
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
Zayd bin Wahb said that Sayyiduna Umar (may Allah be pleased with him) said: In intense heat, one should prostrate on his garment, and when the crowd is heavy, one should prostrate on the back of his brother.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجمعة / 5630