It was reported that the Messenger of Allah, peace and blessings of Allah be upon him, forbade letting one’s clothes hang down during prayer or for a man to cover his mouth. (Reported by Abu Dawood, no. 643).
In ‘Awn al-Ma’bood (2/347) al-Khattaabi said: “Al-sadl: letting one’s clothes hang down all the way to the ground.”
It was reported in Marqaat al-Mafaateeh(2/236): “Al-sadl is completely forbidden because it has to do with showing off, and in prayer it is even worse.”
The author of al-Nihaayah said: “It means wrapping oneself up in one’s garment, leaving one’s hands inside and bowing and prostrating in it.”
It was said that the Jews used to do this. It was also said that al-sadl meant putting the garment over one’s head or shoulders, and letting its edges come down in front and over one’s upper arms, so that a person will be preoccupied in taking care of it, which reduces khushu, unlike garments that are tied up properly or buttoned, which do not distract the worshipper. These kinds of clothes are still to be found nowadays in some parts of Africa and elsewhere, and in the way some Arabian cloaks are worn, which distract the worshipper and keep him busy adjusting them, retying them if they become loose and so on. This should be avoided.
The reason why it is forbidden to cover one’s mouth was explained by the scholars as being because that prevents a person from reciting Qur’an and doing sujud properly. (Marqaat al-Mafaateeh, 2/236).