Numbers 18:29
Out of all your gifts ye shall offer every heave offering of the Lord, of all the best thereof, even the hallowed part thereof out of it.
Cross-reference
Numbers 18:12
Parallel
Numbers 18:12 defines 'the best' as firstfruits of oil, wine, and grain given to priests, grounding the instruction here.
Exodus 29:28
Parallel
Exodus 29:28 also describes a sacred contribution (terumah) due to priests from peace offerings, paralleling the same concept.