7 But when ye pray, use not vain repetitions, as the heathen do: for they think that they shall be heard for their much speaking. -Matthew 6:7 King James Version
16 Moreover when ye fast, be not, as the hypocrites, of a sad countenance: for they disfigure their faces, that they may appear unto men to fast. Verily I say unto you, They have their reward.
17 But thou, when thou fastest, anoint thine head, and wash thy face;
18 That thou appear not unto men to fast, but unto thy Father which is in secret: and thy Father, which seeth in secret, shall reward thee openly. -Matthew 6:16-18 King James
Jesus is making an assumption that the people are already fasting and giving directions on how to fast WHEN they are fasting not IF they choose to fast.
33 But seek ye first the kingdom of God, and his righteousness; and all these things shall be added unto you. -Matthew 6:33 King James