Albert Barnes Commentary 1 Samuel 14:35

"And Saul built an altar unto Jehovah: the same was the first altar that he built unto Jehovah." — 1 Samuel 14:35 (ASV)

And Saul built ... - That is, from the great stone they had rolled into place to slaughter the oxen and sheep on, he began to build an altar to Yahweh; but he did not finish it (Compare 1 Chronicles 27:24), in his haste to pursue the Philistines that night.