Abu Bakr bin Muhammad bin Amr bin Hazm said that the Prophet (peace and blessings be upon him) wrote a letter to the people of Yemen which also contained: "For the tongue too there is the diyah."
The same is reported from Sayyiduna Mu'adh and Sayyiduna Umar (may Allah be pleased with them both).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الديات / 16248