Numbers 4:19
But thus do unto them, that they may live, and not die, when they approach unto the most holy things: Aaron and his sons shall go in, and appoint them every one to his service and to his burden:
Cross-reference
Numbers 4:4 identifies the 'most holy things' the Kohathites must not approach directly — the very items assigned in 4:19.
Numbers 4:20 directly follows: it adds the prohibition against even looking at the holy things, reinforcing the mortal danger in 4:19.
Numbers 18:3 warns Levites not to come near sanctuary vessels or die — the same mortal danger extended to all Levites serving in 4:19.
2 Samuel 6:6 records Uzzah touching the ark and dying — a direct illustration of the death warned against in Numbers 4:19 for approaching holy things.
Leviticus 16:2 warns Aaron not to enter the Holy Place arbitrarily or die — the same life-or-death principle applied to Kohathites in 4:19.
1 Chronicles 15:2 states only Levites may carry the ark — echoing the assignment of holy things to Kohathites in Numbers 4:19.
Ezra 8:28 consecrates priests and Levites to carry holy vessels — a later parallel to the sanctity and assignment of holy things in Numbers 4:19.