Didache (1st Century) 8:1:
Your fasts must not be identical with those of the hypocrites. They fast on Mondays and Thursdays; but you should fast on Wednesdays and Fridays.
The Shepherd of Hermas (1st or 2nd Century), Similitude 5, Chapter 1:
“God does not desire such an empty fasting. For fasting to God in this way you will do nothing for a righteous life; but offer to God a fasting of the following kind: Do no evil in your life, and serve the Lord with a pure heart: keep His commandments, walking in His precepts, and let no evil desire arise in your heart; and believe in God. If you do these things, and fear Him, and abstain from every evil thing, you will live unto God; and if you do these things, you will keep a great fast, and one acceptable before God.”
The Shepherd of Hermas (1st or 2nd Century), Similitude, Chapter 3:
Having fulfilled what is written, in the day on which you fast you will taste nothing but bread and water; and having reckoned up the price of the dishes of that day which you intended to have eaten, you will give it to a widow, or an orphan, or to some person in want, and thus you will exhibit humility of mind, so that he who has received benefit from your humility may fill his own soul, and pray for you to the Lord. If you observe fasting, as I have commanded you, your sacrifice will be acceptable to God, and this fasting will be written down; and the service thus performed is noble, and sacred, and acceptable to the Lord.
- - -
Find more of what the early Christians thought on my Christian History page!