Matthew 8:4

And Jesus saith unto him, See thou tell no man; but go, show thyself to the priest, and offer the gift that Moses commanded, for a testimony unto them.

Mark's account reveals that the leper disobeyed the Lord's command not to publish the matter (Mark 1:44,45). Christ, on several occasions, made similar requests to conceal such miracles. Examples of this are: the blind men (Matthew 9:30), many who were healed (Matthew 12:16), the disciples to whom he was revealed as the Messiah (Matthew 16:20), those healed by the seaside (Mark 3:12), those who saw the healing of the deaf-mute (Mark 7:36), those witnesses of the healing of the blind man of Bethsaida (Mark 8:26), and others. It may border on speculation to inquire why our Lord thus prohibited certain ones from telling it abroad, and yet on other occasions he even encouraged it. Trench has this:

The injunction to one, that he should proclaim, to another that he should conceal, the great things which God had wrought for him, had far more probably a deeper motive, and grounded itself on the different moral conditions of the persons healed.[1]

Trench also noted a practical reason in the case at hand. For the miracle to be properly attested, it was necessary that the appropriate gifts should be offered after Moses' commandment and that the priests should certify it.

Until this was accomplished, he should hold his peace; lest, if a rumor of these things went before him, the priests at Jerusalem, out of envy, out of a desire to depreciate what the Lord had done, might deny that the man had ever been a leper, or else that he was now truly cleansed.[2]

[1] Richard C. Trench, Notes on the Miracles (Westwood, New Jersey: Fleming H. Revell Company, 1953), p. 237.

[2] Ibid., p. 238.