John Gill Commentary 1 Samuel 21:9

"And the priest said, The sword of Goliath the Philistine, whom thou slewest in the vale of Elah, behold, it is here wrapped in a cloth behind the ephod: if thou wilt take that, take it; for there is no other save that here. And David said, There is none like that; give it me." — 1 Samuel 21:9 (ASV)

And the priest said, the sword of Goliath the Philistine, whom
you slew in the valley of Elah
See (1 Samuel 17:2 1 Samuel 17:51) ;

behold, it [is here] wrapped in a cloth behind the ephod ;

the garment of the high priest, in which were the Urim and Thummim, and the breastplate, or the linen vestments of the priests; see (1 Samuel 22:18) ; which were laid up in a chamber for their use; and behind them the sword of Goliath was wrapped up in a linen cloth, and reserved as a monument of the goodness of God to Israel, and the salvation of them wrought by the hands of David, who slew Goliath with this his own sword, and brought it with him.

The Targum understands this word, rendered "behind", not of the place where the sword was, but of the time when the priest said this, and paraphrases the words, ``after he had inquired for him by the ephod;'' see (1 Samuel 22:10) ;

if you want to take that, take [it] ;
as if he should say, it is not mine to give you, but you may take it if you please; none has a better right to it; it is what you took from the Philistine, and may take it again for your use if you are so inclined; and you must either take this or none:

for [there is] no other save that here ;
in the tabernacle, nor even in the priest's house, nor in the city; for it was a city of priests, who did not wear swords:

and David said, [there is] none like that, give it me ;
and which, as he was capable of wielding and making use of, as it is plain he was by cutting off Goliath's head with it; so it might serve to strengthen his faith in God, as often as he looked upon it, that he would keep and preserve him, and in due time deliver him out of the hands of Saul, and all his enemies.