Is covering both shoulders entirely obligatory during prayer?
Chapter on the Description of Prayer
Al-Mughni
Book of Prayer
Primary text
It is sufficient to cover only a portion of the shoulders. Furthermore, covering them with a thin garment that reveals the color of the skin is permissible. The obligation for covering them is derived from the Hadith stating: 'A man should not pray in a single garment with nothing covering his shoulders.' This applies generally, covering both or not all of them. Imam Ahmad, when questioned about a man praying with one shoulder uncovered, did not mandate a repetition of the prayer.
Supporting text
If one places a rope or thread upon the shoulders, the apparent view of Al-Kharqi suggests this is insufficient because it is not considered 'a thing of garment' (shay'an min al-libas). This view is held by Al-Qadi.