In the name of Allah, Most Gracious, Most Merciful.
Abu-Darda (May Allah be pleased with him) said: The Prophet (May Allah exalt his mention and protect him from imperfection) said, “He who defends the honour of his (Muslim) brother, Allah will secure his face against the Fire on the Day of Resurrection.” (At-Tirmidhi)
Commentary: If someone says something disgraceful against a Muslim, one must defend his Muslim brother (or sister, for that matter) and say that what has been stated about him or her is wrong and that he (or she) is free from the accusation made against him (or her).
Riyaadiss-Saliheen (The Meadows of the Righteous)
By Imam An Nawawi
Chapter 255, Page 438, No 1528