Between you and the Father
The Pharisees had distorted the Law to suit themselves. What really matters is the attitude of the heart. They appeared very religious, but it was to be seen.
Jesus told us to do our good deeds in secret; between us and the Father. Our prayer should be focused on the Father.
Calling God Father was revolutionary. The Lord's Prayer puts God first, only then our own needs. Do we ever fast as Christians? Perhaps at times of crisis or before some enterprise. But do it in secret.