I came to the Messenger of Allah ﷺ with a group from Mazina, and we pledged allegiance to him while the buttons of his shirt were undone. So I pledged allegiance to him, then I put my hand inside the opening of his shirt and touched the seal of prophethood.Urwa said, "I never saw Muawiya nor his son except with their shirt buttons undone, whether in winter or summer. They never fastened their buttons at all."
حدثنا النفيلي وأحمد بن يونس قالا حدثنا زهير حدثنا عروة بن عبد الله قال ابن نفيل ابن قشير أبو مهل الجعفي
أتيت رسول الله ﷺ في رهط من مزينة فبايعناه وإن قميصه لمطلق الأزرار قال فبايعته ثم أدخلت يدي في جيب قميصه فمسست الخاتم قال عروة فما رأيت معاوية ولا ابنه قط إلا مطلقي أزرارهما في شتاء ولا حر ولا يزرران أزرارهما أبدا
Its isnad is sahih. Ibn Majah narrated it in "the Book of Clothing, chapter on the permissibility of buttons," no. 3578, and Tirmidhi in Shamail, no. 59 (in my edition), from the hadith of Zuhayr bin Muawiya, through this route, and Ibn Hibban graded it sahih.