Compare Matthew 5:15 and Sifre Numbers 93

"... Literal parallels to passages in rabbinic literature are sometimes important for both sources. Thus we read in the Sermon on the Mount ... In the late midrashic compilation, Midrash ha-Gadol, in which ancient midrashic sources have been preserved, we read ... The medieval compilation does not name the midrashim from which its exegetical comments were derived. Quite often early sources can be reconstructed only on the basis of this compilation. The source just cited as a parallel to Matthew may be a tannaitic saying, although this is by no means certain. This striking parallel demonstrates the antiquity of the rabbinic source. It seems that Matthew 5:16b is an elaboration of an earlier Jewish sentence. In the parallel to our verse, Luke 11:33, the lamp metaphor seems to refer to Jesus' ministry. The Christological context in Luke is much closer to the context in the Jewish source, where the lamp metaphor is applied to 'a prophet'; the wording of the saying in Luke, however, has little in common with our midrash. It is plausible, then, that there was a pre-Matthean source whose context was different from the existing redaction of the Sermon on the Mount, but whose wording was similar to the wording in Matthew ..."