It is narrated from Sayyiduna Ibn Abbas (may Allah be pleased with them both) that indeed the Prophet (peace and blessings be upon him) said: "Whoever looks into his brother's book (or writing) without his permission, it is as if he is looking into the fire."
Hadith Referenceمسند الشهاب / 464
Hadith Gradingمحدثین:إسناده ضعيف
Hadith Takhrijإسناده ضعيف ، ابومقدام ہشام بن زیاد متروک ہے ۔