Blessing over prayer

Why is there no blessing over the mitzvah of prayer?

There are several explanations for this.

1. The Meiri (on tractate Brachos, folio 12b) writes that the prayer itself is a blessing, therefore there is no need to say a blessing over it.
2. The Magen Avraham commentary (on the Shulchan Oruch, section Orach Chaim, ch. 106, subsection 4) writes that the reason is that there is no set time for prayer. The Pri Megadim (a commentary on the Magen Avraam) explains that he refers to this statement of the Sages: “we wish that he should pray the whole day”.
3. Maharam Shik (Responsa, section Orach Chaim, ch. 39) explains that, since a prayer has to be said with proper concentration and intention, there is always a chance that it won’t be said the correct way.