It is narrated marfu‘an from Hazrat Abu Hurairah (may Allah be pleased with him) that at the moment when a person steals, he is not a believer; at the moment when a person drinks alcohol, he is not a believer; and at the moment when a person commits adultery, he is not a believer.
Hadith Referenceمسند احمد / مسند المكثرين من الصحابة / 7318