Matthew 9:9-13 (NIV)
9 As Jesus went on from there, he saw a man named Matthew sitting at the tax collector's booth. "Follow me," he told him, and Matthew got up and followed him.

10 While Jesus was having dinner at Matthew's house, many tax collectors and "sinners" came and ate with him and his disciples.
11 When the Pharisees saw this, they asked his disciples, "Why does your teacher eat with tax collectors and 'sinners'?"
12 On hearing this, Jesus said, "It is not the healthy who need a doctor, but the sick.
13 But go and learn what this means: 'I desire mercy, not sacrifice.' For I have not come to call the righteous, but sinners."

Matthew 9:12 (NIV)
12 On hearing this, Jesus said, "It is not the healthy who need a doctor, but the sick.

“The phrase “go and learn” was commonly used in rabbinic writings to rebuke those who did not know what they should have known. Jesus used the Pharisees' own most honored authorities to rebuke them for their ignorance of God's true nature and of their failure to follow His clear commandments.”

MacArthur New Testament Commentary, The - MacArthur New Testament Commentary

Matthew 9:13 (NIV)
13 But go and learn what this means: 'I desire mercy, not sacrifice.' For I have not come to call the righteous, but sinners."