Rashi on Leviticus 27:26/רש"י על ויקרא כ״ז:כ״ו

1לא יקדיש איש אתו [ONLY THE FIRSTBORN OF BEASTS…] NO MAN SHALL SANCTIFY IT for the purpose of (to become) a sacrifice of any other description, for it is not his, [being already holy to the Lord from birth] (cf. Arakhin 29a).