Midrash Rabbah Shir haShirim on Words of Torah in the Heart

" 'The heart of a wise man enlightens his mouth, and adds learning on his lips' (Proverbs 16:23). The heart of a wise man is full of wisdom. Who enlightens others about him? Who informs others about him? His mouth. When he speaks words of wisdom, his mouth informs others of him; his mouth apprises others of him. '... and adds learning on his lips'. Because the wise man extracts the words of Torah from his heart and enunciates them with his lips, he adds to the learning of Torah."

-- Midrash Rabbah, Shir haShirim 1.6