Sforno on Numbers 6:8/ספורנו על במדבר ו׳:ח׳

1'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.