Musannaf Ibn Abi Shayba 3:2885

Abu Bakr narrated to us, he said: Ibn Mahdi narrated to us, from Abu 'Awanah, from Layth, from Tawus, that he used to dislike returning the greeting and saying "Yarhamuk Allah" (may Allah have mercy on you) to the one who sneezes while the Imam is delivering the sermon.