The practice of daily, set prayer goes back to the Old Testament. The Psalms speak of prayer in the morning (5:3), early hours (130:6), evening (141:2), and day and night (92:2). Psalm 119:164a says, "Seven times a day I praise you." Scripture also mentions thrice-daily prayers (Ps. 55:17, Dan. 6:10). Jews said the Shema (a Scripture-based prayer praising God's greatness) two or three times a day.
Emulating Jewish prayer, early Christians prayed often. The Gospels and Acts report that Jesus and the disciples prayed alone, in synagogues, and in the Temple. We find shared daily prayer as early as Acts 1:14 and 2:42-47. Prayers had set times (Acts 3:1) and set content (though parts were also extemporaneous). A persistent New Testament phrase inspired all prayer: "pray without ceasing" (5:16-18; cf. Mt. 7:7-12; Lk. 11:5-13,18:1; Col. 4:2; Eph. 6:18; 1 Thess. 1:2).
The early church encouraged morning and evening prayer (which included the Lord's Prayer and praying the Psalms). The Didache (perhaps as early as A.D. 60) dictated that the Lord's Prayer be said three times a day, in imitation of Jewish prayers. "We should do in order everything that the Lord commanded us to do at set times," Clement of Rome said in about A.D. 96. "He has ordered oblations and services to be accomplished, and not by chance and in disorderly fashion but at set times and hours."
Clement of Alexandria (A.D. 150-215) and Origen (A.D. 185-254) refer to prayer three times a day. Tertullian, Cyprian, and Hippolytus (all third century) refer to more times of prayer. By the fourth century, many churches had daily public morning and evening prayers. Regular attendance was expected. Ambrose of Milan (339-397) wanted all Christians to attend each morning.