Whenever something troubled the Prophet ﷺ, he would take hold of his beard like this. And Ibn Mushir took hold of his own beard to demonstrate.
أن النبي ﷺ كان إذا همه شيء، أخذ بلحيته هكذا، وقبض ابن مسهر على لحيته.
No references are available.
Ibn Hibban narrated it in his Sahih (6439) from the hadith of Ali bin Mushir, from Muhammad bin Amr bin Alqama bin Waqqas al-Laythi, from his father, from his grandfather, and so he related it. Abu Muhammad Amr bin Alqama was not deemed reliable by anyone other than Ibn Hibban, and for this reason Hafiz said, "Acceptable," meaning when corroborated, and it has indeed been corroborated.Abu Shaykh narrated it in Akhlaq Nabi ﷺ (p. 52) through another route, from Muhammad bin Amr bin Alqama, from Yahya bin Abdul-Rahman bin Hatib, from Ayesha, who said, "When the Messenger of Allah ﷺ felt intense grief, he would touch his beard more." Yahya bin Abdul-Rahman bin Hatib is reliable, having been declared trustworthy by Nasai, Daraqutni, and others.