While there are a number of passages which speak about the Son of Man and while their are many passages which show that thhis was a term that Jesus particularly enjoyed using for himself, there is no one particular place that you can go and find the answer. But there is an explanation.
It has to do with the Hebrew concept of the kinsman redeemer. Leviticus 25:25 Ruth 1-4 Several types are given in the Book of Ruth.
Naomi left the land and went out to exile picturing Israel's departure from God.
Ruth pictures the Gentile bride of Christ.
Boaz is a picture of Christ, the Kinsman Redeemer;
The brother of Boaz is a picture of the law that cannot redeem
The "kinsman redeemer" is a Goel. The word means to redeem, receive or buy back. Provision was made in the Law of Moses for the poor person who was forced to sell part of his property or himself into slavery. His nearest of kin could step in and "buy back" what his relative was forced to sell (Leviticus 25:48f). The kinsman redeemer was a rich benefactor, or person who frees the debtor by paying the ransom price. "If thy brother be waxen poor, and hath sold away some of his possession, and if any of his kin come to redeem it, then shall he redeem that which his brother sold." (Leviticus 25:25; cf. Ruth 4:4, 6).
The nearest of kin had the responsibility of redeeming his kinsman's lost opportunities. If a person was forced into slavery, his redeemer purchased his freedom. When debt threatened to overwhelm him, the kinsman stepped in to redeem his homestead and let the family live. If a family member died without an heir the kinsman gave his name by marrying the widow and rearing a son to hand down his name (Deuteronomy 25:5; Genesis 38:8; Ruth 3-4).
Jesus is and was fully G-d, but in order to redeem sinners He had to become qualified to be the Kinsman redeemer. For this to happen He had to be fully man as well, and free from the curse of sin that condemned mankind. This is the reason that the term Son of Man seems to be Jesus' favorite term to use for Himself.