A.T. Robertson Commentary Luke 19:40

"And he answered and said, I tell you that, if these shall hold their peace, the stones will cry out." — Luke 19:40 (ASV)

If these shall hold their peace (εαν ουτο σιωπησουσιν). A condition of the first class, determined as fulfilled. The use of εαν rather than ε cuts no figure in the case (1 Thessalonians 3:8; 1 John 5:15). The kind of condition is determined by the mode which is here indicative. The future tense by its very nature does approximate the aorist subjunctive, but after all it is the indicative.

The stones will cry out (ο λιθο κραξουσιν). A proverb for the impossible happening.