John Gill Commentary Isaiah 1:15

"And when ye spread forth your hands, I will hide mine eyes from you; yea, when ye make many prayers, I will not hear: your hands are full of blood." — Isaiah 1:15 (ASV)

And when you 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 (Lamentations 3:42)

yea, when you 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, (Matthew 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.