We read in Lk 10: 25-28 :
Just then a lawyer stood up to test Jesus. “Teacher,” he said, “what must I do to inherit eternal life?” He said to him, “What is written in the law? What do you read there?” He answered, “You shall love the Lord your God with all your heart, and with all your soul, and with all your strength, and with all your mind; and your neighbor as yourself.” And he said to him, “You have given the right answer; do this, and you will live.
We also read in Mtt 22: 35-40 :
One of them, a lawyer, asked him a question to test him. “Teacher, which commandment in the law is the greatest?” He said to him, “‘You shall love the Lord your God with all your heart, and with all your soul, and with all your mind.’ This is the greatest and first commandment. And a second is like it: ‘You shall love your neighbor as yourself.’ On these two commandments hang all the law and the prophets.
Let us presume that the lawyers in Luke 10 and Matthew 22 are two different individuals. The one in Luke tells Jesus the summary of the Commandments while it is Jesus who tells the lawyer in Matthew , the summary verbatim. One possibility is that the lawyer in Matthew heard the summary from Jesus and reported it to the lawyer in Luke who would later repeat it to Jesus in response to the Lord's quiz. But then, it is also possible that both the Lord and the lawyer in Mathew had a common source for the summary. But we do not find such a summary in the Old Testament in one place, even though Deut 6:5 (You shall love the LORD your God with all your heart, with all your soul, and with all your strength) and Leviticus 19:18 (… you shall love your neighbor as yourself …) separately mention them, that too, not in the nature of essence of the Commandments.
My question therefore is: According to Catholic Church, what is the original source of summary of the 10 Commandments, which Jesus quotes in Mtt 22: 35-40?