1 Samuel 14:32-33 (NASB)
32 "The people [a]rushed greedily upon the spoil, and took sheep and oxen and calves, and slew them on the ground; and the people ate them with the blood. 33 Then they told Saul, saying, “Behold, the people are sinning against the Lord by eating with the blood.” And he said, “You have acted treacherously; roll a great stone to me today.”
The Mosaic Law says:
Leviticus 17:10-12 (NET Bible)
Prohibition against Eating Blood
10 “‘Any man[a] from the house of Israel or from the resident foreigners who live[b] in their[c] midst who eats any blood, I will set my face against that person who eats the blood, and I will cut him off from the midst of his people,[d] 11 for the life of every living thing[e] is in the blood.[f] So I myself have assigned it to you[g] on the altar to make atonement for your lives, for the blood makes atonement by means of the life.[h] 12 Therefore, I have said to the Israelites: No person among you is to eat blood,[i] and no resident foreigner[j] who lives among you is to eat blood.