Sayyidah Asma' bint Yazid (may Allah be pleased with her) narrates from the Prophet (peace and blessings be upon him) that he (peace and blessings be upon him) said: "Whoever defends the honour of his brother in his absence, it is a right upon Allah, the Mighty and Majestic, to free him from the Fire."
Hadith Referenceالمعجم الكبير للطبراني / مسند النساء / 20814