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