What was the sign of the covenant between God and Abraham?

Answer: Circumcision

Every male was to be circumcised as a sign of the covenant.

“This is My covenant with you and your descendants after you, which you are to keep: Every male among you must be circumcised. You are to circumcise the flesh of your foreskin, and this will be a sign of the covenant between Me and you.”— Genesis 17:10-11 (BSB)