וַיֹּ֣אמֶר לָהּ֩ אֶלְקָנָ֨ה אִישָׁ֜הּ עֲשִׂ֧י הַטּ֣וֹב בְּעֵינַ֗יִךְ שְׁבִי֙ עַד־גָּמְלֵ֣ךְ אֹת֔וֹ אַ֛ךְ יָקֵ֥ם יְהוָ֖ה אֶת־דְּבָר֑וֹ וַתֵּ֤שֶׁב הָֽאִשָּׁה֙ וַתֵּ֣ינֶק אֶת־בְּנָ֔הּ עַד־גָּמְלָ֖הּ אֹתֽוֹ׃
And Elkanah her husband said unto her: ‘Do what seemeth thee good; tarry until thou have weaned him; only the LORD establish His word.’ So the woman tarried and gave her son suck, until she weaned him.
Sforno
'קדוש הוא לה. He will merit the light available for those enjoying eternal life, as well as be able to instruct the spiritual leaders of his generation. This may have been what Elkanah referred to when he said: “may the Lord keep His word” (Samuel I 1,23). What he meant was that he agreed with Chanah his wife’s vow that the child she would bear would spend his life on earth as a Nazirite. He did not add any additional request regarding the child that would be born to them. If G’d would allow Chanah’s prayer and vow to come true this would be more than enough satisfaction for him.
Rashbam
We find a similar expression in connection with Samuel I 1,23 referring to the day he was weaned. Such days were marked by a feast. Chanah brought Samuel to Eli in Shiloh at the time. (Samuel I 1,24)