In addition to the special prayers that our forefathers prayed during times of trouble, they also fixed set times when they prayed to Hashem (Berachot 26b). Avraham Avinu initiated the Shacharit morning prayer. He was the one who originally illuminated the world with his belief, and accordingly set the time of his prayer when the sun starts to rise. Yitzchak Avinu founded the Minchah afternoon prayer. Yitzchak had the unique ability to continue in the way of Avraham his father. Sometimes it is easier to break away onto a new path rather than carry on in the same one. Yitzchak’s strength was that he remained in the path of faith, corresponding to the Minchah prayer, which expresses continuity, for the whole day is sustained by the power of faith. Yaakov Avinu formulated the Ma’ariv evening prayer because Yaakov dealt with many hardships and complications, yet from each of them he emerged stronger. He therefore established the nighttime prayer, since, even in the dark when reality is clouded, it is possible to connect to HaKadosh Baruch Hu, thereby revealing the supreme eternal light.
After the forefathers paved the way with these prayers, there were devout and righteous people who followed in their path and prayed Shacharit, Minchah, and Ma’ariv. As King David said (Psalms 55:17-18), “As for me, I call out to the Lord, and God saves me. Evening, morning, and noon, I express my grief and moan aloud, and He hears my voice.”
Following in the custom of our forefathers, Anshei Knesset HaGedolah established the three prayers, Shacharit and Minchah as obligatory, and Ma’ariv as optional. They were set up to correspond to the communal offerings, since the prayers came to express the inner significance of the sacrifices. Since the prayers were established to correspond to the sacrifices, the times of the prayers were set according to the times of the offerings (as explained further in this book 11:4, 11:11, 24:3-4, and 25:2).
Because the Tamid sacrifices of the morning and of the afternoon were obligatory, Shacharit and Minchah are obligatory prayers. Ma’ariv was established to represent the burning of the fat and organs, which were put on the altar at night. If one did not bring them, it did not prevent him from fulfilling the mitzvah of the offering. Therefore Ma’ariv was also deemed optional. However, as time passed, the nation of Israel took upon itself to recite Ma’ariv as an obligatory prayer (see further in this book 25:2). On Shabbat, festivals, and Rosh Chodesh, we were commanded to bring a Musaf sacrifice; hence, the Chachamim established the recital of the Musaf prayer to represent it.