Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) transmits this saying of the Prophet (peace and blessings be upon him): "Whoever does not show mercy is not shown mercy."
Al-Bazzar and al-Tabarani recorded it; its chain contains a narrator, Atiyyah, who has been declared trustworthy despite his weakness; the remaining narrators of al-Bazzar are the narrators of the Sahih.