Zaid says that one day he went to Amir’ul-Mu’minin (a.s) and saw a pot of water kept near him and on the other side was a canvas bag with its mouth sealed. “I thought he would remove precious things from it and give them to me. When the Imam broke the seal and opened the bag I saw that it contained parched barley meal. He took out a handful, mixed it in a cup of water, offered to me and took some himself. I could not restrain myself, and I said, ‘Sir, living in Iraq you have such food? While different kinds of eatables are available here?’ He said, ‘This is sufficient for survival.’ I said, ‘Why do you keep the bag sealed?’ He replied, ‘So that my family may not mix oil etc. in it. I do not want anything to be a part of my diet except barley.’”
It is written in Sharh Nahjul Balagha that the Imam always observed a diet of vinegar and salt. If he exceeded in it he ate some vegetables and if he further increased it, he drank some camel milk. He used to eat very little meat and he said, “Do not make your stomach a graveyard of animals.”
Reference :Akhlaq al-A’imma, Morals & Manners of the Holy Imams