Jesus says something very close to that in
Matthew 11:11 and
Luke 7:28.
Here's the wording in both passages:
* Matthew 11:11 (NIV): "Truly I tell you, among those born of women there has not risen anyone greater than John the Baptist; yet whoever is least in the kingdom of heaven is greater than he."
* Luke 7:28 (NIV): "I tell you, among those born of women there is no one greater than John; yet the one who is least in the kingdom of God is greater than he."
Note that in both passages, Jesus qualifies his statement by adding that those in the Kingdom of Heaven/God are greater than John. This is often interpreted as referring to those who have accepted Jesus as the Messiah and are part of the new covenant.