There is no question Christ said it, as we find it in several places..
Mark 12:27
He is not the God of the dead, but the God of the living: ye therefore do greatly err.
Luke 20:38
For he is not a God of the dead, but of the living: for all live unto him.
So lets begin in Matthew..
Matthew 22:23-33
23 The same day came to him the Sadducees, which say that there is no resurrection, and asked him,
24 Saying, Master, Moses said, If a man die, having no children, his brother shall marry his wife, and raise up seed unto his brother.
25 Now there were with us seven brethren: and the first, when he had married a wife, deceased, and, having no issue, left his wife unto his brother:
26 Likewise the second also, and the third, unto the seventh.
27 And last of all the woman died also.
28 Therefore in the resurrection whose wife shall she be of the seven? for they all had her.
29 Jesus answered and said unto them, Ye do err, not knowing the scriptures, nor the power of God.
30 For in the resurrection they neither marry, nor are given in marriage, but are as the angels of God in heaven.
31 But as touching the resurrection of the dead, have ye not read that which was spoken unto you by God, saying,
32 I am the God of Abraham, and the God of Isaac, and the God of Jacob? God is not the God of the dead, but of the living.
33 And when the multitude heard this, they were astonished at his doctrine.
Look at what Jesus said to the Sadducees in verses 30-32: "For in the resurrection they neither marry nor are given in marriage, but are like angels in heaven. And as for the resurrection of the dead, have you not read what was said to you by God, 'I am the God of Abraham, the God of Isaac, and the God of Jacob?' He is God not of the dead, but of the living" .
The whole passage if you look is a discussion between Jesus and the Sadducees about the resurrection. They denied that there would be a resurrection of the dead and came to see what Christ would say. They posed a hypothetical case of a woman who married seven brothers in turn and asked Jesus which of the brothers would be her husband after the resurrection. Jesus' answer focused on the resurrection: "For in the resurrection they neither marry nor are given in marriage, but are like the angels in heaven" in verse 30. Then he added: "But as for the resurrection of the dead, have ye not read what was said to you by God, I am the God of Abraham, the God of Isaac, and the God of Jacob? He is God not of the dead, but of the living". What did Christ mean?
