Musnad Ahmad 30:2534

Yahya ibn Sa'id narrated to us, from Ibn 'Ajlan, who said: Sumayy narrated to me, from Abu Salih, from Abu Hurayrah, who said: "When the Messenger of Allah (peace and blessings of Allah be upon him) sneezed, he would place his hand or his garment over his forehead, and he would lower or muffle his voice."