It is narrated from Sayyiduna Sahl bin Sa'd (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "Beware of minor sins, for the example of minor sins is like that of a people who camped in a valley; one brought a piece of wood and another brought one too, until they gathered enough wood with which they baked their bread. And indeed, when minor sins seize a person, they destroy him."
Hadith Referenceالمعجم الكبير للطبراني / باب السين / 5851