The Messenger of Allah ﷺ said, "Do not drape the walls. Whoever looks into his brother's letter without his permission is only looking into the Fire. Ask Allah with the palms of your hands, not with their backs, and when you finish, wipe your faces with them."
حدثنا عبد الله بن مسلمة حدثنا عبد الملك بن محمد بن أيمن عن عبد الله بن يعقوب بن إسحق عمن حدثه عن محمد بن كعب القرظي
أن رسول الله ﷺ قال لا تستروا الجدر من نظر في كتاب أخيه بغير إذنه فإنما ينظر في النار سلوا الله ببطون أكفكم ولا تسألوه بظهورها فإذا فرغتم فامسحوا بها وجوهكم
[Its chain of transmission is daif] Bayhaqi reported it, 2/212, from the hadith of Abu Dawud through the same chain, and it contains an unknown narrator as well as another defect. The hadith has daif corroborating narrations in Ibn Majah, no. 3866, and elsewhere. As for his saying, "Do not cover the walls," it is hasan, having a corroborating narration in Tahawi's Maani al-Athar, 4/283.