I
"And Avraham arose early in the morning, to the place - that he had stood there before Hashem.” (19:24)
The Gemara (Brachot 26b) following the opinion of Rabbi Yossi b’Rabbi Chanina, demonstrates that the phrase “that he had stood” refers to prayer. Similarly, the Gemara finds such a source by both Yitzchak Avinu and Yaakov Avinu. These references serve as the basis for Rabbi Yossi b’Rabbi Chanina’s opinion that the three daily prayers - Shacharit, Mincha, and Arvit - were instituted by the Avot.
Rabbi Yehoshua ben Levi (ibid.) disagrees. Rather, he maintains, the three prayers are of later origin; they were composed by the Anshei Knesset Hagedola (the Great Assembly), corresponding to the sacrificial services in the Beit Hamikdash. [The Gemara concludes that even according to Rabbi Yossi b’Rabbi Chanina, the laws regarding the various zmanei tefilla (proper times to daven) are based upon the korbanot in the Beit Hamikdash. see ibid.]
II
Clearly, the aforementioned opinions deal with only a Rabbinic obligation of prayer. Neither the Avot nor the Anshei Knesset Hagedola can create a chiyuv mi'dorayta - a scriptural obligation. As such, we can safely conclude that our daily obligation to pray Shacharit, Mincha and Maariv is a chiyuv mi'drabbonon - Rabbinic obligation.
Is there a chiyuv mi'dorayta to pray at least once a day? Perhaps, only the number of three prayers plus the standardized text of our shmoneh esrei are of rabbinic origin, but nonetheless, a chiyuv mi'dorayta (albeit less rigorous) does exist, demanding that we pray some sort of prayer at least once a day.
This question is a source of dispute between Rambam and Ramban. Rambam (Sefer Hamitzvot 5; Hilchot Tefillah, Chapter 1) maintains that in fact there is such a chiyuv d'orayta. Based on Chazal, he claims that various psukim which demand us to “serve Hashem”, refer specifically to an obligation of daily prayer. Yet, Ramban explains otherwise (Sefer Hamitzvot, ibid.), claiming that there is no d'orayta obligation of daily prayer; it is totally midrabbonon.
III
The Mishna in Brachot (20b) rules that women are obligated to pray daily. At first glance, this seems rather odd. After all, prayer is a classical example of a mitzva shehazman gramah - a mitzva that is governed by time [morning, afternoon, evening] - which generally exempts women?
According to Rambam, the answer is simple. True, the d'rabbonon obligation to pray three times -Shacharit, Mincha, Arvit - is zman Gramah, and women are in fact exempt from this obligation. The Mishna refers instead, to the d'orayta obligation of prayer - once a day, any time of day. This obligation is obviously not zman gramah and thus, women too are obligated to pray once a day, some sort of valid prayer but not necessarily shmoneh esrei. [See Rambam Hilchot Tefillah, ibid., as to what constitutes a “valid” prayer mi'd'orayta. See also Magen Avraham O.C. 106:2 And Pri Megadim ibid.]
Yet, according to Ramban the question remains. There exists only one level of obligation regarding our daily prayers, rabbinic in nature, to daven three times a day. If so, the Mishna is clearly obligating women to daven all the daily prayers. Why? Isn’t this a Mitzva Shehazman Gramah?
The answer is that according to Ramban tefillah is “rachamei” - “a request for mercy”. [Our texts of the Gemara have this reason stated explicitly in the Gemara Brachot 20b.] Women need to make requests of Hashem just as men. As such, Chazal required women to daven - just as men - despite tefilla’s status as zman Gramah. [See Magen Giborim O.C. 106 for another explanation.]
IV
The contrast between Rambam’s understanding of the Mishnah in Brachot - versus Ramban’s - is striking and of much significance in the daily life of a woman. According to Ramban every woman must daven a shmoneh esrei for Shacharit and for Mincha! [Regarding Arvit, Mishna Brurah rules leniently since “tefillat Arvit reshut”. (M.B. 106:4) We discussed this status of Arvit last year, Parshat Vayeitzei.] According to Rambam though, a less standardized text uttered once a day suffices.
Although some rule leniently like the opinion of Rambam, the majority follow that of Ramban. And so rules the Mishna Brurah (ibid.).
[Note: Our discussion relates to shmoneh esrei. The laws regarding all other parts of tefillah - such as psukei d'zimrah, birchot kriat shema, shema, etc. - are beyond our present scope.]