Leviticus 20:15
And if a man lie with a beast, he shall surely be put to death: and ye shall slay the beast.
Cross-reference
Leviticus 18:23
Parallel
Leviticus 18:23 prohibits lying with an animal — the very act that Leviticus 20:15 then punishes with death for both man and beast.
Exodus 22:19
Parallel
Exodus 22:19 also commands death for anyone who lies with an animal — a direct parallel to the same penalty here.
Deuteronomy 27:21
Parallel
Deuteronomy 27:21 pronounces a curse on anyone who does this, reinforcing the same prohibition with a communal 'Amen'.