من لا يحضره الفقيه, volume 1, page 310

Hadith.926 - In what al-Fadl narrated from Imam al-Ridha (as) concerning reasons for the recitation in prayer, he stated: “People have been commanded to recite during prayer so that the Qur’an would not be abandoned or neglected, but instead preserved and studied so that it is neither forgotten nor lost. The recitation begins with Surah al-Hamd (The Opening) because it uniquely contains comprehensive wisdom and blessings not found in other verses of the Qur’an. The phrase ‘Alhamdulillah’ (All praise is due to Allah (swt)) is a fulfilment of the obligation to thank Allah (swt) and an expression of gratitude for enabling His servant to do good. ‘Rabbil 'Alamin’ (Lord (azj) of the Worlds) affirms Allah’s (swt) unity, praise, and recognition that He is the Creator and Owner without any partner. ‘Ar-Rahman Ar-Rahim’ (The Most Merciful, The Most Compassionate) is a plea for Allah’s (swt) kindness and a reminder of His blessings upon all creation. ‘Maliki Yawmid-Deen’ (Master of the Day of Judgment) is a declaration of belief in the resurrection, accountability, and Allah’s (swt) dominion over both worlds. ‘Iyyaka Na’budu’ (You alone we worship) signifies devotion and sincerity to Allah (swt) in worship. ‘Wa iyyaka nasta’in’ (And You alone we seek for help) seeks His continued assistance and favour. ‘Ihdinas-Siratal-Mustaqeem’ (Guide us to the Straight Path) asks for steadfastness in religion, adherence to divine guidance, and further recognition of Allah (swt). ‘Siratal-Ladheena an’amta ‘alayhim’ (The path of those whom You have blessed) expresses longing for the grace granted to Allah’s (swt) chosen ones. ‘Ghayril-maghdubi ‘alayhim’ (Not the path of those who have evoked [Your] anger) seeks protection from becoming among those who oppose or belittle Allah’s (swt) commands. ‘Wala-dhallin’ (And not of those who have gone astray) and [it is] a protection from being among those who have gone astray from His path without knowledge, thinking they are doing good. In this, all the essentials of goodness and wisdom, from both the matters of the world and the hereafter, have been combined in a way that nothing else can gather." [AL SADUQ] He the reason why loud recitation (Jahr) is prescribed in some prayers and not in others is explained. The prayers in which recitation is loud are done during dark times, so it is necessary to recite loudly so that people passing by can know that a congregation is present. If they want to pray, they can join. Without loud recitation, it would be hard for those passing by to know there is a congregation. The prayers in which recitation is not loud are performed in the daytime when it's bright, so they can be known through sight, and there is no need for hearing. When you recite the Al-Fatiha and a Surah, you should say Takbir once while standing, then bow and place your right hand on your right knee before your left, with your palms on your knees. Spread your fingers around the knee joint, and extend your neck, keeping your gaze directed between your feet, toward the place of prostration."

Translation: Bab Ul Qaim Publications — attributed by Thaqalayn·The Arabic above is the original.