In historical-critical analysis, the L source is an inferred oral tradition that Luke used when composing his gospel. It includes the Christmas story and many of Jesus' best loved parables. Like Matthew's unique source, known as M-Source

M-Source

M-source, which is sometimes referred to as M document, or simply M, comes from the M in "Matthean material". It is a hypothetical textual source for the Gospel of Matthew...

, the L source has important parables that may be authentic to the historical Jesus. Two that appear in L are the Good Samaritan and the Prodigal Son.

According to the Four Document Hypothesis, Luke combined Mark, the Q source

Q source

The Q source is a hypothetical written source for the Gospel of Matthew and Gospel of Luke. Q is defined as the "common" material found in Matthew and Luke but not in the Gospel of Mark...

, and L to produce his gospel. The material in L, like that in M, probably comes from the oral tradition. Luke's special material composes nearly half of his gospel.

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.