The Messenger of Allah ﷺ forbade a man from covering his mouth during the prayer.
نهى رسول الله ﷺ أن يغطي الرجل فاه في الصلاة.
Ibn Majah (966) narrated it from Abu Saeed Sufyan bin Ziyad Al-Muaddib, who said, "Muhammad bin Rashid narrated to us, from Hasan bin Zakwan, from Ataa, from Abu Hurairah," and he mentioned it. Hasan bin Zakwan is disputed, but his hadith is hasan.Abu Dawud (642) narrated it from Muhammad bin Ala and Ibrahim bin Musa, from Ibn Mubarak, from Hasan bin Zakwan, from Sulayman Al-Ahwal, from Ataa, and Ibrahim said, "from Abu Hurairah," that the Prophet ﷺ "forbade sadl in prayer, and that a man cover his mouth."See the full referencing of this in the chapter on the prohibition of sadl in prayer.