THE SYNOPTIC PROBLEM
No history has ever been written that records dialogue. It is impossible to comprehensively do so. For no one knows the importance of an event until well afterwards. The Gospels comprehensively record the dialogue of events
Not all witnesses see and hear the same things and so different aspects of an event are easily recorded by different witnesses.
Large parts of the Gospels are dialogue and oratory. A good orator uses repetition.
Hence disagreement must be expected, as an idea is repeated in different forms.
Sources for Gospel material may be many. Such as individual Gospel writers, bystanders, healed people, Temple records, or the obvious one, Jesus himself. Jesus spent 40 days with the disciples before his ascension, in which time they could easily have written down all that he wanted them to say. Humanly speaking this is the obvious source for the Gospels.
The Holy Spirit is said to be the source for it is said.
the Comforter, the Holy Ghost, … he.. .shall… bring all things to your remembrance, whatsoever I have said unto you. Jn. 14:26
flesh and blood hath not revealed it unto thee, but my Father which is in heaven. Mt. 16:17
It must be said that some events had no notable witnesses other than Christ. There were no disciples other than John at the religious trials of Christ. Yet the other writers record them and that different to John and each other. Therefore, there was no literary dependence, copying, there. And other writers were, at least in this instance, not the source of any one other.
There were events recorded where no followers of Christ were present e.g. the temptations.
There is agreement in basic order of each Gospel. This largely rules out an oral source, which would have few chronological indicators.
A brief overview of the generally accepted views on the Synoptic Problem are given in these links. If you have little understanding of the problem it would pay to look at these sites first, which generally give an accepted view of the situation, but which is at variance to what is contained on this website. bible.org/article/synoptic-problem and www.blueletterbible.org/faq/q.cfm.
PAPIAS was the first to say that there was an oral collection of sayings of Jesus before the Gospels were written.
This implies that the Gospels could not be chronogically written, nor accurately transmitted.
This assumed collection of sayings on Papias' testimony, whether in written or oral form is the basis for the assumption that there is a
Q source. (German quelle -source)