Musannaf Ibn Abi Shayba 21:444

Waki' narrated to us, from Sufyan, from al-Jurayri, from Abu al-Ward, from Ibn A'bad or Ibn Ma'bad, who said: Ali said, "Do you know what the right of food is?" I said, "And what is its right?" He said, "You say: 'In the name of Allah, O Allah, bless for us that which You have provided for us.'" Then he said, "Do you know what its gratitude is?" I said, "And what is its gratitude?" He said, "You say: 'Praise be to Allah who has fed us and given us drink.'"