Dr.Lightfoot writes in his note upon the passage as follows: 'As Clement's quotations are often very loose, we need not go beyond the Canonical Gospels for the source of this passage.

The resemblance to the original is much closer here, than it is for instance in his account of Rahab above, Sec.

12.
The hypothesis therefore that Clement derived the saying from oral tradition, or from some lost Gospel, is not needed.' (1) No doubt it is true that Clement does often quote loosely.

The difference of language, taking the parallel clauses one by one, is not greater than would be found in many of his quotations from the Old Testament.

(2) Supposing that the order of St.Luke is followed, there will be no greater dislocation than e.g.in the quotation from Deut.ix.