Didache (1st Century) 8:3:
[after quoting the Lord’s prayer] “You should pray in this way three times a day.”
Didache (1st Century) 10:1-7:
After you have finished your meal, say grace in this way:
"We thank you, holy Father, for your sacred name which you have lodged in our hearts, and for the knowledge and faith and immortality which you have revealed through Jesus, your child. To you be glory forever.
"Almighty Master, 'you have created everything' for the sake of your name, and have given men food and drink to enjoy that they may thank you. But to us you have given spiritual food and drink and eternal life through Jesus, your child.
"Above all, we thank you that you are mighty. To you be glory forever.
“Remember, Lord, your Church, to save it from all evil and to make it perfect by your love. Make it holy, 'and gather' it 'together from the four winds' into your Kingdom which you have made ready for it. For yours is the power and the glory forever. "
"Let Grace come and let this world pass away."
"Hosanna to the God of David!"
"If anyone is holy, let him come. If not, let him repent."
"Our Lord, come!"
In the case of prophets, however, you should let them give thanks in their own way.
The Shepherd of Hermas (1st or 2nd Century), Vision 3, Chapter 1:
[The angel speaking] “Hermas, cease praying continually for your sins; pray for righteousness, that you may have a portion of it immediately in your house.
The Shepherd of Hermas (1st or 2nd Century), Commandment 9:
He [the angel] says to me, “Put away doubting from you and do not hesitate to ask of the Lord, saying to yourself, ‘How can I ask of the Lord and receive from Him, seeing I have sinned so much against Him? ‘Do not thus reason with yourself, but with all your heart turn to the Lord and ask of Him without doubting, and you will know the multitude of His tender mercies; that He will never leave you, but fulfil the request of your soul. For He is not like men, who remember evils done against them; but He Himself remembers not evils, and has compassion on His own creature. Cleanse, therefore, your heart from all the vanities of this world, and from the words already mentioned, and ask of the Lord and you will receive all, and in none of your requests will you be denied which you make to the Lord without doubting. But if you doubt in your heart, you will receive none of your requests. For those who doubt regarding God are double-souled, and obtain not one of their requests. But those who are perfect in faith ask everything, trusting in the Lord; and they obtain, because they ask nothing doubting, and not being double-souled. For every double-souled man, even if he repent, will with difficulty be saved. Cleanse your heart, therefore, from all doubt, and put on faith, because it is strong, and trust God that you will obtain from Him all that you ask. And if at any time, after you have asked of the Lord, you are slower in obtaining your request [than you expected], do not doubt because you have not soon obtained the request of your soul; for invariably it is on account of some temptation or some sin of which you are ignorant that you are slower in obtaining your request. Wherefore do not cease to make the request of your soul, and you will obtain it. But if you grow weary and waver in your request, blame yourself, and not Him who does not give to you.
2 Clement (Early to Mid-2nd Century) Chapter 15:
Let us, therefore, continue in that course in which we, righteous and holy, believed, that with confidence we may ask God who saith, "Whilst thou art still speaking, I will say, Here I am." For these words are a token of a great promise, for the Lord saith that He is more ready to give than he who asks.
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Find more of what the early Christians thought on my Christian History page!