It is narrated from Sayyiduna Ibn Umar (may Allah be pleased with them both) that the Messenger of Allah (peace and blessings be upon him) said: "Refrain from (declaring disbelief upon) the people who say 'There is no true god except Allah'; do not declare them disbelievers because of a sin. Whoever declares one who says 'There is no true god except Allah' a disbeliever is himself closer to disbelief."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 13033