The Prophet ﷺ said, "None of you should pray in a single garment with nothing covering his shoulders."
قال النبي ﷺ: لا يصلي أحدكم في الثوب الواحد، ليس على عاتقيه شيء.
Bukhari narrated it in the Book of Prayer (359), and Muslim in the Book of Prayer (516), both through Abu Zinad, from Abdul-Rahman Araj, from Abu Hurairah, and he mentioned it. Muslim added, "something of it." His words, "there is nothing on his shoulders," mean that he should not simply wrap the garment around his waist or tie its two ends at his hips, but rather should drape it over his shoulders so that covering is achieved for part of the upper body, even though that part is not among the private parts requiring covering, or because doing so makes covering the private parts more secure. This is stated in Fath al-Bari.The majority hold that this prohibition is a prohibition of etiquette, for if a person covers what is between his navel and his knee, his prayer is valid, but the sunnah is that he should pray in a waist-wrap and an upper wrap if he has both available. This is stated in Sharh al-Sunnah (2/422), and a hadith of Abu Hurairah will come later supporting this.Imam Ahmad and some of the early scholars said that his prayer is not valid if he is able to place something over his shoulder unless he does so, based on the apparent meaning of the hadith.