In order to fulfill the law of Moses, Jesus told the leper He had been healed to go and
In the context of the New Testament, according to the book of Matthew (8:2-4), Jesus healed a leper who came to him for healing. After healing him, Jesus told him to "go and show himself to the priest and offer the gift that Moses commanded, as a testimony to them."
This statement refers to the Jewish law of Moses, which commanded that a person with leprosy must present himself to a priest for examination after being healed. If the priest confirmed that the person was indeed healed, he would then perform a ritual cleansing and the person would be considered clean.
So, Jesus was telling the leper to follow the Jewish law of Moses by presenting himself to the priest as a testimony to them that he had been healed, which would serve as proof of the miraculous healing that Jesus had performed.