Musannaf Ibn Abi Shayba 33:1409

Abu Usama narrated to us, from Abu al-Ashhab, from al-Hasan, who said: They used to say that the tongue of the wise man is behind his heart. So when he intends to speak, he refers to his heart; if it is for him, he speaks, and if it is against him, he refrains. As for the ignorant man, his heart is at the tip of his tongue; he does not refer to his heart. Whatever comes to his tongue, he speaks it.