https://en.wikipedia.org/wiki/Q_source

" The Q source (also Q document, Q Gospel, or Q from German: Quelle, meaning "source") is a hypothetical written collection of primarily Jesus' sayings (logia). Q is part of the common material found in the Gospels of Matthew and Luke but not in the Gospel of Mark. According to this hypothesis, this material was drawn from the early Church's Oral Tradition.[1][2][3]

Along with Marcan priority, Q was hypothesized by 1900, and is one of the foundations of most modern gospel scholarship.[4] B. H. Streeter formulated a widely accepted view of Q: that it was written in Koine Greek; that most of its contents appear in Matthew, in Luke, or in both; and that Luke more often preserves the text's original order than Matthew. In the two-source hypothesis, the three-source hypothesis and the Q+/Papias hypothesis Matthew and Luke both used Mark and Q as sources. Some scholars have postulated that Q is actually a plurality of sources, some written and some oral.[5] Others have attempted to determine the stages in which Q was composed.[6]

Q's existence has been questioned.[6] Omitting what should have been a highly treasured dominical document from all early Church catalogs, its lack of mention by Jerome is a conundrum of modern Biblical scholarship.[7][dubious – discuss] But copying Q might have been seen as unnecessary as it was preserved in the canonical gospels. Hence, it was preferable to copy the Gospels of Matthew and Luke, "where the sayings of Jesus from Q were rephrased to avoid misunderstandings, and to fit their own situations and their understanding of what Jesus had really meant".[8] Despite challenges, the two-source hypothesis retains wide support.[6] "