Maliks Muwatta Book 1, Hadith Number 31.

Section : The Prohibition against Entering the Mosque Smelling of Garlic and the Prohibition against covering the Mouth in Prayer.

Yahya related to me from Malik from Abd ar-Rahman ibn al-Mujabbar that he used to see Salim ibn Abdullah pull the cloth away fiercely from the mouth of any man he saw covering his mouth while praying.