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 respecting the future destiny of Samuel was made. “The Bath-kol (Daughter of the Voice) went forth, saying, There shall arise a just one, whose name shall 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 fulfil what He designs with him, and has promised by his birth.