Musannaf Ibn Abi Shayba 3:349

Abu Bakr narrated to us, he said: Abu Mu'awiyah narrated to us, from al-A'mash, from al-Musayyab ibn Rafi', from Zayd ibn Wahb, from 'Umar, he said: If a man is unable to prostrate on the day of Friday, then let him prostrate on the back of his brother.