And as touching the dead, that they rise: have ye not read in the book of Moses, how in the bush God spake unto him, saying, I [am] the God of Abraham, and the God of Isaac, and the God of Jacob?
Note 3 at Mr 12:26: The Sadducees accepted only the five books of Moses as authoritative and rejected the oral traditions held by the Pharisees as well as the rest of Old Testament scriptures (The Bible Knowledge Commentary, page 162). Therefore, Jesus referred to the Pentateuch (the five books of Moses) in answering their questions.
Jesus spoke of the burning bush (Ex 3:1-6) in which God identified Himself as the God of the living and not of the dead. Jesus did this by emphasizing the word "am" in God's statement, "I am [not was] the God of thy father" (brackets mine). This was such a simple and yet profound interpretation of this passage that it astonished everyone (Mt 22:33) and even drew respect from His critics (Lu 20:39).
The Sadducean understanding of death was extinction, not separation of the spirit and soul from the body (Jas 2:26), as the Bible teaches (see note 14 at Joh 5:28). Jesus' final remarks in Mr 12:27 showed how greatly deceived they were to deny the resurrection and life after death.