The Messenger of Allah ﷺ forbade letting one's garment hang loosely during prayer and forbade a man from covering his mouth.
أن رسول الله ﷺ نهى عن السدل في الصلاة، وأن يغطي الرجل فاه.
Abu Dawud (643) narrated it from Muhammad bin Ala and Ibrahim bin Musa, from Abdullah bin Mubarak, from Hasan bin Zakwan, from Sulayman Ahwal, from Ataa. Ibrahim said, from Abu Hurairah, and he mentioned the hadith. Its chain is hasan due to the discussion surrounding Hasan bin Zakwan, though he is hasan al-hadith. Ibn Adi said, "He narrates hadiths that no one else narrates, and I hope there is no harm in him." Ibn Hibban mentioned him among the trustworthy narrators. His teacher Ibn Khuzayma (772) graded this hadith sahih, transmitting it through the route of Abdullah bin Mubarak in the same manner.Abu Dawud said, "Asl narrated it from Ataa, from Abu Hurairah, that the Prophet ﷺ forbade sadl during prayer." I say, this is a corroborating narration for what preceded, and Tirmidhi (378) connected it, saying, Hannad told us, Qabisa told us, from Hammad bin Salama, from Asl bin Sufyan, from Ataa (who is Ibn Abu Rabah), in the same manner. Tirmidhi said, "We do not know the hadith of Abu Hurairah from the hadith of Ataa, from Abu Hurairah, as a raised hadith, except through the hadith of Asl bin Sufyan." I say, the matter is not as he said, rather what preceded contradicts it, except that Asl bin Sufyan is daif, but the corroboration of Hasan bin Zakwan raises the hadith to the level of hasan.As for Hakim (1/253), he narrated it through the route of Husayn bin Zakwan, from Ahwal, and graded it sahih according to the standard of the two sheikhs (Bukhari and Muslim). Husayn bin Zakwan is al-Muallim, who is trustworthy and well known among the narrators accepted by the community, and he is different from Hasan bin Zakwan Abu Salama, who is disputed. Perhaps this became confused for Hakim, so he graded it sahih according to the standard of the two sheikhs. Bayhaqi (2/242) got it right, narrating it from Abdullah bin Mubarak, from Hasan bin Zakwan, in the same manner. Ibn Majah (966) narrated it correctly, but only the second half.Sadl means letting the garment hang down until it touches the ground, and it carries the same meaning as isbal. This is how Khattabi explained it. In al-Nihaya it is said that sadl means wrapping oneself in one's garment while putting one's hands inside it, then bowing and prostrating in that state, and this is what the Jews used to do.As for his saying, "and that a man cover his mouth," Khattabi said, "It was the custom of the Arabs to cover their mouths with their turbans, so they were forbidden from doing that, except when a person praying is overcome by a yawn, in which case he may give his mouth room at that point, according to the hadith that has come regarding this."