Sayyiduna Anas (may Allah be pleased with him) narrates from the Prophet (peace and blessings be upon him) that he (peace and blessings be upon him) said: "The greatest of the major sins are associating partners with Allah, killing a soul unjustly, disobedience to parents, and false speech, or false testimony."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الشهادات / 20829
Abu al-Malih al-Hudhali (may Allah have mercy on him) said: Sayyiduna Umar (may Allah be pleased with him) wrote a letter to Sayyiduna Abu Musa al-Ash'ari (may Allah be pleased with him), and he mentioned the hadith, in which it says: "Muslims are upright (as witnesses) for one another, except one who has been flogged in a hadd punishment, or one known by experience to have given false testimony, or one under suspicion regarding clientship (wala') or kinship."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الشهادات / 20830