In the Musnad, Ibn Mas‘ūd reported:
“They used to recite behind the Prophet, may Allah bless him and grant him peace, so he said, ‘You have confused me in my recitation of the Quran.’”
This expresses his disapproval of someone competing with him, contending with him, and disrupting his recitation of the Quran. This cannot happen when someone recites quietly to himself so that no one else hears him; it happens only when he makes himself audible to others. This is disliked because it involves competing with someone else, not because he is reciting behind the imam. As for reciting when the imam recites quietly, no hadith has been reported prohibiting this. Hence his question:
“Which of you was reciting?”
He meant the person whose recitation competed with his, not someone reciting quietly to himself. Such a person does not compete with him, nor is he known to have contended with the Prophet, may Allah bless him and grant him peace. Reciting behind the imam is disliked only when it prevents the attentive silence that is commanded, or when it involves competing with someone else. If there is neither a command to listen silently nor any competing recitation, there are no grounds for prohibiting the recitation of the Quran during prayer. Here, the person reciting has not been given listening as a substitute for recitation; thus, if prevented from reciting, he would miss out on both listening and reciting. There is, moreover, a well-known disagreement over whether recitation is obligatory in such circumstances. By contrast, the view that it is obligatory when the imam recites aloud is anomalous; Ahmad even reported a consensus to the contrary.
Abū Hurayra and other Companions understood His words:
“I have divided the prayer into two halves between Myself and My servant. When the servant says, ‘All praise is for Allah, Lord of all worlds’”
to apply to both the imam and the person praying behind him. Moreover, every remembrance that the imam is prescribed to say quietly is also prescribed for the person praying behind him