Virtue of Speaking up for a Muslim in his absence

From Abu'l-Darda' (RA): The Messenger of Allah (SAAS) said: "Whoever defends the honour of his brother, this will be a protection for him from the Fire." [Al-Khara'iti, Makarim al-Akhlaq; Ibn 'Asakir. There is nothing wrong with its Isnad as there is corroborating evidence. See Ghayah al-Maram (below Hadith No. 431).]

From Asma' bint Yazid (RA), who said: The Prophet (SAAS) said: "Whoever defends his brother's honour in his absence, will be entitled to Allah's protection from the Fire." [Ahmad et al. It is Sahih because of corroborating and similar reports. See Ghayah al-Maram (431).] 

The Prophet (SAAS) said: "Whoever defends his brother's honour, Allah will protect his face from the Fire of the Day of Resurrection."[Narrated and classed as Hasan by al-Tirmidhi. Classed as Hasan by al-Albani, Ghayah al-Maram (below Hadith 431).]

From Mu'adh ibn Anas al-Juhani: The Prophet (SAAS) said: "Whoever protects a Muslim from a hypocrite, (probably he said:) Allah will send an angel to protect his flesh from the Fire of Hell; and whoever accuses a Muslim of something, seeking to disgrace him, Allah will detain him on the bridge of Hell until he has been fully punished for what he said." [Abu Dawud: Sahih Sunan Ab iDawud (4086); Ibn Abi Dunya. See also al-Albani, Sahih al-Targhib wa'l-Tarhib.]

Courtesy: www.everymuslim.net