Abu Dawud Book 27, Hadith Number 4071.
Chapter : Opening the buttons of the collar.
Narated By Qurrah ibn Iyas al-Muzani : I came to the Apostle of Allah (PBUH) with a company of Muzaynah and we swore allegiance to him. The buttons of his shirt were open. I swore allegiance to him and I put my hand inside the collar of his shirt and felt the seal.
Urwah said: I always saw Mu’awiyah and his son opening their buttons of the collar during winter and summer. They never closed their buttons.