Q source
Scholars doubt that the authors were the evangelists Matthew and Luke: it seems unlikely, for example, that Matthew would rely so heavily on Mark if its author had been an eyewitness to Jesus's ministry, or that the Acts of Apostles (by the same author as the gospel of Luke) would so frequently contradict the Pauline letters if its author had been Paul's companion. Instead, the two took for their sources the gospel of Mark (606 of Matthew's verses are taken from Mark, 320 of Luke's), the Q source, and the "special" material of M and L.