Rashi on Numbers 18:24/רש"י על במדבר י״ח:כ״ד

1אשר ירימו לה׳ תרומה [BUT ALL THE TITHES OF THE CHILDREN OF ISRAEL] WHICH THEY HEAVE AS A HEAVE OFFERING UNTO THE LORD — Scripture terms it (the tithe) T’rumah (although it is not holy and may be eaten by a layman) until he (the Levite) sets apart from it the “heave offering of the tithe” (Sifrei Bamidbar 119:5).