The parallel here is not close. But Matthew has a doublet in xxi, 21, and Mark a similar saying in xi, 22. The saying seems therefore to have been in both Mark and Q, and was taken by Matthew from both sources and by Luke from one. The connection of the saying in Luke indicates that he took it from Q; yet his saying is not the same as Matthew's, in that he substitutes a sycamore tree for Matthew's mountain, thus greatly weakening the comparison. The two sayings certainly cannot have been derived by Matthew and Luke from an identical source. It is only on the ground of their general logian character that they can be assigned to QMt and QLk.
The comparison here is complicated by the fact that this saying apparently stood in both Mark and Q. It is closely, but in reverse order by the two later evangelists, connected with a saying taken from Mark. It may be assigned to Q.
There seems here to be little or no literary relationship. The two passages appear to be rather different versions of the same parable, which have come down thru different channels. If it be assumed that Matthew's version is from Q, there is not enough literary agreement between it and Luke's to prove the latter to be from any recension of that document. Considering the larger content of Matthew's recension, and his apparently greater unwillingness to make omissions from it, it might be safe to assign this to QMt, but to leave Luke's source for his version unspecified. At the same time it is well to remember that the parables stand apparently half-way between the narratives and the sayings, as regards the willingness of the evangelists to deviate from the wording found before them. If enough may be allowed for this difference between parables and sayings, the divergence between the two Gospels in this section might not be considered too great to be accounted for by the known habits of Matthew and Luke, working on different recensions of an original Q; and so the passage might be assigned to QMt and QLk--but certainly not with any confidence.
These might be considered merely as variants of the same original saying. If the reference to Peter be taken, like some of the other references to him in Matthew, to be later than the saying itself, the insertion of this reference in Matthew, whether by Matthew or his source, may have changed the form of the saying from its original as preserved in Luke. But the very slight verbal agreement makes any specification of a common literary source hazardous.