In historical-critical analysis, the L source is an inferred oral tradition that Luke used when composing his gospel.[1] It includes the Christmas story and many of Jesus' best loved parables. Like Matthew's unique source, known as M-Source, the L source has important parables that may be authentic to the historical Jesus.[1] Two that appear in L are the Good Samaritan and the Prodigal Son.[1]
According to the Four Document Hypothesis, Luke combined Mark, the Q source, and L to produce his gospel.[1] The material in L, like that in M, probably comes from the oral tradition.[1] Luke's special material composes nearly half of his gospel.[2]
The question of how to explain the similarities among the Gospels Matthew, Mark, and Luke is known as the synoptic problem. The hypothetical L source fits a contemporary solution in which Mark was the first gospel and Q was a written source for both Matthew and Luke.