John Gill Commentary Hebrews 9:20

"saying, This is the blood of the covenant which God commanded to you-ward." — Hebrews 9:20 (ASV)

Saying, this is the blood of the testament
The first testament or covenant; this proves what the apostle has asserted in (Hebrews 9:18) that it was dedicated with blood, or confirmed by it; compare with this (Matthew 26:28)

which God has enjoined to you ;
the people of Israel, to observe, and which they promised to do; see (Exodus 24:7Exodus 24:8) .