Musannaf Ibn Abi Shayba 3:343

Abu Bakr narrated to us, he said: Hushaym narrated to us, he said: Mujalid informed us, from al-Sha'bi, from Sa'id ibn Dhi Lu'wah, he said: Umar said: "If one of you is unable to prostrate on the day of Friday, then let him prostrate on the back of his brother."