Albert Barnes Commentary 2 Samuel 23:16

"And the three mighty men brake through the host of the Philistines, and drew water out of the well of Beth-lehem, that was by the gate, and took it, and brought it to David: but he would not drink thereof, but poured it out unto Jehovah." — 2 Samuel 23:16 (ASV)

Broke through the host - Their camp was pitched in the valley of Rephaim (2 Samuel 23:13; 1 Chronicles 11:15). It follows from this that the way from Adullam to Bethlehem lay through or across the valley of Rephaim.

Poured it out to the Lord - It was too costly for his own use; none but the Lord was worthy of it. For libations, see the note on Judges 6:20.