Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) said that Muhammad (peace and blessings be upon him) said: "Shall I not tell you what al-'idah (slander and tale-bearing) is?" He said: "It is tale-bearing that is carried on between people." And the Prophet (peace and blessings be upon him) said: "A man, by always speaking the truth, is counted with Allah among the truthful; and a person, by lying, is written with Allah among the liars."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الشهادات / 21158