Flavius Josephus, Antiquities of the Jews
William Whiston, A.M., Ed.

[120] When, therefore, they had slain many ten thousands of the Philistines, they fell upon spoiling the camp of the Philistines, but not till late in the evening. They also took a great deal of prey and cattle, and killed them, and ate them with their blood. This was told to the king by the scribes, that the multitude were sinning against God as they sacrificed, and were eating before the blood was well washed away, and the flesh was made clean. Then did Saul give order that a great stone should be rolled into the midst of them, and he made proclamation that they should kill their sacrifices upon it, and not feed upon the flesh with the blood, for that was not acceptable to God. And when all the people did as the king commanded them, Saul erected an altar there, and offered burnt-offerings upon it to God ¹ This was the first altar that Saul built.