Maliks Muwatta Book 49, Hadith Number 27.
Section : General Section on Food and Drink.
Yahya related to me from Malik that he had heard that Isa ibn Maryam used to say, “O Banu Israil! You must drink pure water and the green things of the land and barley bread. Beware of wheat bread, for you will not be grateful enough for it.”