Abu Dawud

Hadith Commentary:
(1)
This hadith is clear evidence that the Messenger of Allah (sallallahu alayhi wa sallam) would recite quietly in the prayers of Dhuhr and Asr, because Khabbab bin Al-Aratt (radi Allahu anhu) has reported this regarding him and considered the movement of the beard as its sign.
This hadith has already been mentioned previously.
(‘Umdat al-Qari: 4/495) (2)
It is also understood from this hadith that the blessed beard of the Prophet (sallallahu alayhi wa sallam) was long and thick.

Hadith Commentary:
From here, the chapter heading is derived.
Because how could they have known that the beard was moving without looking at the imam?
In any case, during the prayer, one should keep one's gaze either on the imam or at the place of prostration, and should not glance here and there.

Hadith Commentary:
(1)
Some individuals have stated that merely the movement of the beard does not provide conclusive evidence for recitation, because it only indicates that something is being read; it is possible that supplication (du'a) or remembrance (dhikr) is being recited. However, the Companion Khabbab ibn al-Aratt radi Allahu anhu considered the movement of the beard as an indication of recitation, because he had seen the Messenger of Allah sallallahu alayhi wa sallam during the audible prayers, and this is the place for recitation, not for supplication or remembrance.
Along with this, when the statement of Abu Qatadah radi Allahu anhu is included—that the Messenger of Allah sallallahu alayhi wa sallam would sometimes let us hear a verse—indeed, in some narrations, it is explicitly mentioned that he recited verses from Surah Luqman and Surah adh-Dhariyat.
And in the narration of Ibn Khuzaymah, there is clarification of the recitation of: ﴿سَبِّحِ اسْمَ رَبِّكَ الْأَعْلَى ﴿١﴾ and ﴿هَلْ أَتَاكَ حَدِيثُ الْغَاشِيَةِ ﴿١﴾.

(2)
From these evidences, it is understood that the movement of the beard here indicates recitation specifically.
Imam al-Bukhari rahimahullah also deduced from this hadith that the Messenger of Allah sallallahu alayhi wa sallam would recite quietly (sirr) in the Dhuhr and Asr prayers.
Imam al-Bayhaqi rahimahullah has written that for silent recitation, it is necessary that the tongue and lips move and that a person can hear himself.
If one reads with the lips closed and without moving the tongue, the beard does not move as a result.
And Allah knows best.

Benefits and Issues:

➊
Recitation (qira’ah) takes place in all prayers, both silent (sirrī) and audible (jahrī).
Abu Hurairah radi Allahu anhu said: (Fī kulli ṣalāh yuqra’u famā asma‘anā Rasūlullāh sallallahu alayhi wa sallam asma‘nākum wa mā akhfā ‘annā akhfaynā ‘ankum) (Ṣaḥīḥ al-Bukhārī, al-Adhān, Chapter: Recitation in Fajr, Hadith: 772)
“Recitation occurs in every prayer.
Whatever the Messenger of Allah sallallahu alayhi wa sallam made us hear, we make you hear.
And whatever the Noble Prophet sallallahu alayhi wa sallam concealed from us, we conceal from you.
That is, in those rak‘ahs in which the Messenger of Allah sallallahu alayhi wa sallam recited aloud, we also recite aloud.
And in those prayers or rak‘ahs in which he sallallahu alayhi wa sallam recited silently, we also recite silently.”

➋
In silent prayers and rak‘ahs, the manner of recitation is that the lips should be moved in accordance with the words.
Merely reading in the heart, without any movement of the lips, is not sufficient.

➌
If one’s gaze is raised towards the imam during prayer, it does not invalidate the prayer.

➍
In silent prayers, the noble Companions radi Allahu anhum inferred that the Messenger of Allah sallallahu alayhi wa sallam was reciting by observing the movement of his blessed beard.
Additionally, sometimes when a portion of a verse was recited audibly, the Companions radi Allahu anhum would become aware of his recitation in this way as well.
See: (Ṣaḥīḥ al-Bukhārī, al-Adhān, Chapter: Recitation in ‘Asr, Hadith: 762)

Benefit:
From this hadith, it is established that the follower (muqtadi) may look towards the imam during prayer. Additionally, it is necessary to recite (the Qur'an) in the Zuhr and Asr prayers, and it is also proven that movement of the beard during prayer does not cause any deficiency in the prayer. If there is a beard, it will move. It is regrettable that the Muslim Ummah has disrespected the beard and, by chasing after fashion, has abandoned an obligation. Keeping the beard is obligatory; the command of obligation demands it. The beard was obligatory in all previous religious laws.