Tosefta על תרומות 3:5

[If] someone was going to separate terumah or first tithe or poor man's tithe, when does he bless on them? When he begins separating. One who separates, if he will be designating in the future, it has no holiness until he [actually] designates them, and if he will not be designating in the future, then as soon as he separates it becomes holy.