"But this kind is not cast out but by prayer and fasting." -[Matthew 17:20].
"For the space of forty days; and was tempted by the devil. And he ate nothing in those days;" - [Luke 4:2]
"That thou appear not to men to fast, but to thy Father who is in secret: and thy Father who seeth in secret, will repay thee."-[Matthew 6:18]
"Orthodox Abbot Rafailo of Monastery Podmaine in Montenegro, tells you how to answer if someone asks you - why do you fast?"
Source: Orthodox Church - "Orthodox Answer - If someone asks you why do you fast" https://youtu.be/suhtmkn_l20