It is narrated from Sayyiduna Imran bin Husayn (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) said: "What do you think about the fornicator, the thief and the one who drinks wine? What do you say about them?" The Companions said: Allah and His Messenger know best. He (peace and blessings be upon him) said: "These are shameful deeds, and there is punishment for them. Shall I not tell you what the greatest of the major sins is? Associating partners with Allah." Then he (peace and blessings be upon him) recited this verse: "And whoever associates partners with Allah has certainly fabricated a tremendous sin" [al-Nisa: 48]; and disobedience to parents. Then he said: "Be grateful to Me and to your parents; to Me is the final return" [Luqman: 14]. He (peace and blessings be upon him) had been reclining, then he sat upright and said: "Beware! And false speech." Ibn Abbas (may Allah be pleased with them both) said: Everything that Allah has forbidden is a major sin.
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 15384