Matthew 6:16

“Moreover when you fast, do not be, as the hypocrites, of a sad expression,

For they disfigure their faces,

That they may be seen of men to fast.

Truly I say to you, They have received their reward.

Jesus clearly here expects that His disciples will at some time engage in fasting, although He nowhere actually encourages it, even though He anticipates that they will fast once He has been taken from them, presumably with grief (Matthew 9:15). He had, of course given an example of it when He faced up to His own temptations (Matthew 4:1). There the purpose of the fasting had been in order to ensure no interruption in His communion with His Father. Consider also 1 Corinthians 7:5 where abstinence from sex is described for the purpose of devotion to a season of prayer. But He warns them that if and when they fast, it should be secretly so as not to be noticeable. Otherwise they will already have received their reward in terms of the honour that they will receive for it.

‘They disfigure their faces.' This may indicate simply not washing and shaving, or oiling their heads, or it may even signify putting ashes on am making themselves look interesting.