From
The Didache, chapter 8.
1 But as for your fasts, let them not be with the hypocrites, for they fast on the second and fifth days of the week, but do ye fast on the fourth and sixth days.
2 Neither pray ye as the hypocrites, but as the Lord hath commanded in his gospel so pray ye: Our Father in heaven, hallowed be thy name. Thy kingdom come. Thy will be done as in heaven so on earth. Give us this day our daily bread. And forgive us our debt, as we also forgive our debtors. And lead us not into temptation, but deliver us from the evil: for thine is the power, and the glory, for ever.
3 Thrice a day pray ye in this fashion.