Abu ad-Darda' (RAA) narrated that the Messenger of Allah (ﷺ) said:
"If a Muslim defends his brother's honor in his absence, Allah will protect his face from the fire of Hell on the Day of Resurrection." Related by At-Tirmidhi who graded it to be Hasan. (IT)
English reference (IT): Book 16, Hadith 1571 (IT)
Arabic reference (IT): Book 16, Hadith 1528 (IT)