Charles Ellicott Commentary 1 Samuel 1:23

"And Elkanah her husband said unto her, Do what seemeth thee good; tarry until thou have weaned him; only Jehovah establish his word. So the woman tarried and gave her son suck, until she weaned him." — 1 Samuel 1:23 (ASV)

Only the Lord establish his word. —No special word or promise of the Eternal in the case of the infant Samuel is recorded in this history; but there was an ancient Rabbinical tradition that a direct revelation concerning the future destiny of Samuel was made. “The Bath-kol (Daughter of the Voice) came forth, saying, There will arise a just one, whose name will be Samuel. Then every mother who bore a son called him Samuel; but when they saw his actions, they said, This is not Samuel. But when this one was born, they said, This is that Samuel, and this is what the Scripture means when it says, ‘The Lord confirmed his word that Samuel may be that just one.’”— Rashi.

If we decline to accept the Rabbinical tradition, Bunsen’s simple comment will explain the difficult words of the text, establish his word: that is, may the Lord fulfill what He designs for him, and has promised by his birth.