In the same episode of Abram's

[Mary Naeem]

Pope Shenouda III

In the same episode of Abram's meeting with Melchizedek, we hear that Abram "gave him a tithe of all" (Gen.14: 20). How was it known, at the time of our Father Abraham, that the tenth were to be given to priests, except through Tradition? The Law of tithing had not yet been received in the written Law.
This also applies to our father Jacob: How did he know the idea of giving the tithe when he said to the Lord: "... and of all that You give me I will surely give a tenth to You" (Gen.28: 22)? There is no doubt that he received the Law of tithing through Tradition, from his grandfather Abraham who offered the tithe to Melchizedek without receiving it from a written Law at all. It is obvious that Tradition was the teacher of all human beings before the written Law and remained so thereafter.