Rashi on Exodus 29:28/רש"י על שמות כ״ט:כ״ח

1לחק עולם מאת בני ישראל AS AN ASSIGNED PORTION FROM THE CHILDREN OF ISRAEL — because peace-offerings belong to the “owner”, and the breast and the shoulder shall they give to the priest.

2כי תרומה הוא FOR IT IS A HEAVE OFFERING — i.e. the breast and the shoulder form a heave-offering.