Jesus assumed His disciples would fast as He assumed they would give alms and pray. He said nothing to discourage them from fasting (cf. Matthew 9:14-17). He only condemned ostentatious fasting. To avoid any temptation to pander to the adulation of onlookers Jesus counseled His disciples to do nothing that would attract attention to the fact that they were fasting when they fasted. Again, the Father who sees the worship that His children offer in secret will reward them. [source][source][source]
The three major acts of Jewish worship-alms-giving, prayer, and fasting-were only representative of many other acts of worship that Jesus" disciples performed. His teaching in this section of the Sermon ( Matthew 6:1-18) stressed lessons they should apply more broadly. In His teaching about each of these three practices, Jesus first warned His disciples not to do the act for man"s praise. Then He assured them that if they disregarded His warning they would get human praise but no more. Third, He taught them how to do the act secretly. Finally, He assured them that the Father who sees in secret would reward their righteous act openly. He thereby explained what it means to seek first the kingdom and its righteousness ( Matthew 6:33). [source][source][source]