(a) Salim bin Abdullah (may Allah have mercy on him) narrates from his father that the Messenger of Allah (peace and blessings be upon him) mentioned something about the waist-wrapper, and Abu Bakr (may Allah be pleased with him) said: "O Messenger of Allah! My waist-wrapper slips down on one side." He (peace and blessings be upon him) said: "You are not one of them."
(b) It has been reported to us from Ibn Umar (may Allah be pleased with them both), Salim bin Abdullah (may Allah have mercy on him), Sa'id bin al-Musayyib (may Allah have mercy on him), al-Sha'bi (may Allah have mercy on him), Ikrimah (may Allah have mercy on him) and Ibrahim al-Nakha'i (may Allah have mercy on him) that all of them considered covering the face in prayer to be disliked (makruh), and the narration of al-Hasan bin Dhakwan explicitly states its prohibition.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 3315