Musannaf Ibn Abi Shayba 33:965

Abu Usama narrated to us, from Yazid and Abu al-Ashhab, from al-Hasan, who said: It used to be said: The heart of the believer is behind his tongue; so if he intends a matter, he contemplates it. If it is good, he speaks it, and if it is other than that, he remains silent. The heart of the hypocrite is upon the tip of his tongue; so if he intends a thing, he speaks it and reveals it.