Luke 5:14

The healing of lepers was a messianic act (cf. Luke 7:22). Therefore the man’s "testimony" to his cleansing amounted to an announcement of Messiah’s arrival. Jesus did not want this man to fail to go to Jerusalem and present the required offering for the healing of leprosy (Leviticus 14:1-32). If the man had broadcast his healing, he may never have reached the priests there and the crowds may have mobbed Him even worse than they were already doing.