Musannaf Ibn Abi Shayba 18:208

Abu Bakr narrated to us, he said: 'Abbad bin al-'Awwam narrated to us, from 'Umar bin 'Amir, from Makhul, from Zayd bin Thabit, he said: He said regarding the tongue: "If it splits and then heals, [the compensation is] twenty camels."