Musannaf Ibn Abi Shayba 3:432

Abu Bakr narrated to us, he said: Ibn Fudayl narrated to us, from Dawud ibn Abi Hind, from Abu Harb ibn Abi al-Aswad, who said: Abu al-Aswad suffered from hemiplegia, so he would not prostrate except that we would raise a cushion for him, which he would prostrate upon. We asked about that, so we sent to Ibn Umar. He said: "If he is able to prostrate on the ground, [he should do so]; otherwise, he should gesture [with his head]."