Abu Ma’mar reported: I said to Khabbab, “Did the Messenger of Allah, peace and blessings be upon him, recite in the noon and afternoon prayers?” Khabbab said yes. We said, “How do you know?” Khabbab said, “By the movement of his beard.”
Source: Ṣaḥīḥ al-Bukhārī 777
Grade: Sahih (authentic) according to Al-Bukhari
Al-Nawawi said, “The minimum of silent recitation is that one hears himself, if he is of sound hearing and there is no distracting noise around him or otherwise. This is general for Quran recitation, exaltation and glorification in bowing or otherwise, the testimony of faith, the salutation of peace, supplication, or anything else, whether it is obligatory or supererogatory. None of it counts unless he can hear himself.”
Source: al-Majmū’ Sharḥ al-Muhadhab 3/295
عَنْ أَبِي مَعْمَرٍ قُلْتُ لِخَبَّابٍ أَكَانَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ يَقْرَأُ فِي الظُّهْرِ وَالْعَصْرِ قَالَ نَعَمْ قُلْنَا مِنْ أَيْنَ عَلِمْتَ قَالَ بِاضْطِرَابِ لِحْيَتِهِ
777 صحيح البخاري أبواب صفة الصلاة باب من خافت القراءة في الظهر والعصر
قال النووي وَأَدْنَى الْإِسْرَارِ أَنْ يسمع نفسه إذا كان صحيح المسع ولا عارض عند من لغط وَغَيْرِهِ وَهَذَا عَامٌّ فِي الْقِرَاءَةِ وَالتَّكْبِيرِ وَالتَّسْبِيحِ فِي الرُّكُوعِ وَغَيْرِهِ وَالتَّشَهُّدِ وَالسَّلَامِ وَالدُّعَاءِ سَوَاءٌ واجبها ونفلها لا يحسب شئ مِنْهَا حَتَّى يُسْمِعَ نَفْسَهُ
3/295 المجموع شرح المهذب