Most of what Jesus is purported to have said has its source in other writings.
In fact, most of the NewTestament is based on pre-existing literary structures such as Homer or early OT tales.
This was common practice as a literary trope or template.
Whatever remains of Jesus' actual teaching would have had to pass muster by copyists, redactors and translators time and time again who were invariably compelled to "help" him make sense (i.e. prove what they themselves believed.)
when a Textual Critic studies manuscript variants there is a rule of thumb which I believe really illustrates what happened.
If there are two stories (one in each variant manuscript) and one of them is confusing BUT the other one is clear---the Rule of Thumb is....
..............wait for it....................wait for it..................the one that DOESN'T make sense is the older of the two.
Why?
Because the copyist always tried to HELP the translation he was making by FORCING "sense" into the story!!
Bottom line?
If Jesus cited outside sources it might have been dropped from his teachings by copyists and translators because it takes attention away from his divinity and supernatural "powers."
Just a thought.