Musannaf Ibn Abi Shayba 3:4924

Ma'n ibn 'Isa narrated to us, from Malik ibn Anas, from 'Abd al-Rahman ibn al-Mujabbar, that Salim ibn 'Abd Allah, when he would see a man covering his mouth while he was in prayer, he would pull the garment with a strong pull until he removed it from his mouth.