Tafsīr al-ʿAyyāshi: From Abī Ḥamza:

From Abī Jaʿfar عليه السلام, who said: The Messenger of Allāh صلى الله عليه وآله would recite In the name of Allāh, The Entirely Merciful, The Especially Merciful (Bismillah al-Raḥmān al-Raḥīm) out loud, and raise his voice with it.1 Therefore, when the polytheists would hear this they turned their backs, so Allāh sent down [the āya]: “And when you mention your Lord alone in the Qur’ān, they turn back in aversion.2

1. According to our scholars, this ḥadīth, and others like it, prove that it’s recommended to recite the basmala out loud in prayers which are read quietly such as the two midday prayers (dhuhr and ʿaṣr).
2. al-‘Isrā’ (17:46)

عن أبي حمزة ، عن أبي جعفر ( عليه السلام ) ، قال : كان رسول الله ( صلى الله عليه وآله ) يجهر ب * ( بِسْمِ اللَّه الرَّحْمنِ الرَّحِيمِ ) * ويرفع صوته بها ، فإذا سمع المشركون ولوا مدبرين ، فأنزل الله : وإِذا ذَكَرْتَ رَبَّكَ فِي الْقُرْآنِ وَحْدَه وَلَّوْا عَلى أَدْبارِهِمْ نُفُوراً