John Gill

And when ye spread forth your hands,.... That is, in prayer, this being a prayer gesture: hence the Targum paraphrases it, "and when the priests spread out their hands to pray for you.'' I will hide mine eyes from you; will not look upon them, nor regard their prayer; see Lam 3:42. yea, when ye make many prayers; as the Scribes and Pharisees did in Christ's time, and thought to be heard for their much speaking, like the Gentiles, Mat 6:7. I will not hear; so as to give an answer, or fulfil their requests: the reason follows, your hands are full of blood; of the prophets of the Lord, of Christ and his followers, whom they put to death.