Fiqh-us-Sunnah Volume 2, Supererogatory Prayer, Fiqh 2.092C.
Section : Covering the mouth and letting one’s garment down until it touches the ground.
Abu Hurairah said: “The Messenger of Allah prohibited assadl in the salah and prohibited a man to cover his mouth.” This is related by the five and by al-Hakim who says that it is sahih according to Muslim’s conditions. Al-Khattabi explains: “As-sadl is to lower one’s garment until it reaches the ground.” Al-Kamal ibn al-Hamam adds: “This also applies to wearing a cloak without putting one’s arms through its sleeves.”