20 Why then dost still think of your flocks? “Shall I eat the flesh of bulls, or shall I drink the blood of he-goats?”. You have heard what of us He requires not, who wills to enjoin us somewhat. If of such things you were thinking, now withdraw your thoughts from such things: think not to offer God any such thing. If you have a fat bull, kill for the poor: let them eat the flesh of bulls, though they shall not drink the blood of he-goats. Which, when you shall have done, He will account it to you, that has said, “If I shall be hungry, I will not tell you:” and He shall say to you, “I was hungry, and you gave Me to eat.” “Shall I eat the flesh of bulls, or shall I drink the blood of he-goats?”
Source: The Enarrations, or Expositions, on the Psalms (New Advent)