This “source” is entirely speculative.  There is no direct evidence for such a document.  Were there other written documents?  Maybe.  I would go so far as to speculate (and it is speculation) that there were one or more written accounts floating around before the first of the gospels (Mark or Matthew) was written.  There certainly was an oral tradition about Jesus.  But there is literally no evidence of the actual invented Q document.  It is the figment of the imagination of scholars.  They look at similarities and differences between Matthew, Mark, and Luke and invent a document out of sheer speculation as an ad hoc creation to explain the similarities and differences.  What is a bit frustrating to me is that scholars talk about this fictional text as if it existed, when it is, in fact, an ad hoc modern creation.  Scholars make statements such as Matthew xx:xx comes from the Q document.  It is a bit like young people talking about superheroes like Spiderman as if he were a real person.  By the way, Spiderman is not a real person.