הלכה על ברכות 60:6

And also from the content of the commandment is that which they warned us much about intent of the heart in prayer. And more [so] in the first blessing, about which they, may their memory be blessed, said (Berakhot 30b) that we make one who did not have intent in it go back [and repeat it]. And the matter of intent that they obligated going back because of its [absence] appears to be that the person puts into his heart that he is praying in front of God and he is calling to Him, and [that] he empty his mind from all other thoughts of the world and focus it on this. And [also] that which they said (Mishneh Torah, Laws of Prayer and the Priestly Blessing 4:5) that there are things that impede a person from praying, even though the time for prayer has arrived. And among them are purity of the hands; covering the nakedness; purity of the place of prayer; and things that rush a person, such as if one needs [to use the restroom]. And [also] that which they said (Mishneh Torah, Laws of Prayer and the Priestly Blessing 5:5) that there are things about which the one praying must be careful, but we do not impede prayer on their account. And these are them: standing; to direct [himself] that he prays facing the Temple; that he fix his body, meaning to say that he stands with awe and fear - with his eyes down and his heart towards the Heavens and he place his hands on his heart, like a slave standing in front of his master; that he fix his clothing and not stand dressed in the way of commoners; that he make his voice even, not too loud and not too soft; and that he bow during the well-known blessings and these are them - during the fathers (the first blessing) at the beginning and at the end, and during thanksgiving (the penultimate blessing), at the beginning and at the end.