Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) said: The Messenger of Allah (peace and blessings be upon him) was asked about waswasah (evil whispering), and he (peace and blessings be upon him) said: "That is a sign of pure faith."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 9988