Didache 14

But on the Lord's day do ye assemble and break bread, and give thanks, after confessing your transgressions, in order that your sacrifice may be pure. But every one that hath controversy with his friend, let him not come together with you, until they be reconciled, that your sacrifice may not be profaned. For this is that which was spoken by the Lord: At every place and time, bring me a pure sacrifice; for a great king am I, saith the Lord, and my name is marvellous among the nations.