The International Q Project reconstructs the original Q saying as follows:
If you have faith like a mustard seed, you might say to this mulberry tree: be uprooted and planted in the sea! And it would obey you.
Despite the wide attestation of this saying (preserved in three different forms and in three independent witnesses), the commentary in [The Five Gospels] (p. 98f) notes that a majority of the Fellows voted the item "gray or black on the grounds that it was a commonplace adopted by Mark and the other evangelists" rather than a saying that originated with Jesus.
Samuel T. Lachs
Lachs [Rabbinic Commentary on the New Testament] (p.263) observes that a specific rabbinic expression oqer harim was used for such feats where doing "the near impossible" was involved. Lachs also cites a rabbinic parallel to miraculously moving a tree (as in Luke 17:6):
May this carob tree be moved, and it was." [B. BM. 58b]
Luedemann [[esus] (p.79) regards this saying as authentic:
This saying is based on a hyperbolic metaphor (cf. Matt. 17.20; Luke 17.6) of the kind that Jesus also loved elsewhere (cf. 10.25). It emphasizes the power of a characteristic faith and is probably authentic. (Cf. Matt. 7.7-11/Luke 11.9-13)
John P. Meier
Meier [Marginal Jew] (II, p.889) comments as follows:
A variant form of the saying on the power of prayer (Mark 11:22-23) to move a mountain (or alternatively, a sycamore tree) is found in the Q material (Matt 7:19-20 || Luke 17:6) and more distantly in Paul's remark on the power of faith to move mountains in 1 Cor 13:2. Obviously, this saying, perhaps rooted in a popular proverb, circulated widely and independently of any larger context in first-generation Christianity (Mark, Q, Paul).