Is there a passage in the bible where Jesus instructs us to fast?
Jesus answered them, “Can the wedding guests mourn as long as the bridegroom is with them? The days will come when the bridegroom is taken away from them, and then they will fast.” (Matthew 9:15)
Jesus, the bridegroom, did not require fasting while He was with his disciples but He clearly indicated they would fast after the Ascension.
When you fast, do not look gloomy like the hypocrites. They neglect their appearance, so that they may appear to others to be fasting. Amen, I say to you, they have received their reward. But when you fast, anoint your head and wash your face, so that you may not appear to others to be fasting, except to your Father who is hidden. And your Father who sees what is hidden will repay you. (Matthew 6:16-18)
In Matthew 6 Jesus says when you fast, not if you fast. Jesus clearly presumed and took for granted that His followers would fast and issued guidelines for fasting.