The obligation to pray is derived from the verse: “To love the Lord your God, and to serve Him with all your heart and with all your soul” (Deuteronomy 11:13). Which is the service of God that is performed in the heart?… This is prayer (Ta’anit 2a). The Rambam maintains that the mitzva is to pray every day (Mishneh Torah, Sefer HaMitzvot, positive commandment 5), whereas the Ramban contends that the mitzva is to pray whenever a person finds himself in times of trouble and feels a need to turn to God (Ramban’s comments on Rambam, Mishneh Torah, Sefer HaMitzvot, positive commandment 5).

The formula of the prayers evolved over time, from the prayers of biblical figures until their consolidation into the Sephardic and Ashkenazic prayer books in use today. The general structure and formula began to take shape in the early Second Temple period, when the members of the Great Assembly composed the Amida prayer (Megilla 17b). The Mishna and Gemara already mention several sections of the prayers, among them the morning blessings, the verses of song from the book of Psalms, the Daily Psalm, Hallel, and the blessings of Shema, but these are not presented in a systematic or precise order, and it is difficult to ascertain the order in which these sections are to be recited.

The complete prayer formula, from beginning to end, dates back to the geonic period, and it first appears in the prayer books of Rav Amram Gaon and Rav Se’adya Gaon (including Kaddish inserted between the various sections, and the blessings before and after the verses of song) and in the order of prayers at the end of Mishneh Torah, Sefer Ahava, of the Rambam. In later generations, the hasidim of Ashkenaz, the Ari and his disciples, and the leaders of the Hasidic movement all continued editing, formulating, and revising on the basis of earlier versions of the prayer book, primarily on the basis of mysticism. They even added certain sections to the prayer book. In that way, the formula of the different prayer books was consolidated.

The obligation to pray three times a day is based on a verse in the book of Daniel: “Three times a day he knelt on his knees” (Daniel 6:11). The time for each of these prayers corresponds to the time of the sacrifice of the fixed offerings brought in the Temple in Jerusalem. The calculations of the correct times of day and night are affected by the position of the earth relative to the sun during the various seasons of the year, as well as the geographic location of the one praying. Consequently, the time for prayer varies from place to place. The appropriate time to pray is influenced by the situation of the specific individual in question as well: His employment, his involvement in public affairs, the place where he is located, his ability to pray with proper intent, and his proximity to a place of prayer. In accordance with the above variables, one is often obligated to pray slightly earlier or later than the prescribed times of the fixed prayers.