Musannaf Ibn Abi Shayba 28:872

'Abbad ibn al-'Awwam narrated to us, from Abu 'Atiyyah, who said: 'Umar wrote to the people of Kufa: "It has been mentioned to me that 'matras' in the Persian language means 'do not fear.' So if you say it to someone who does not understand your language, he is safe."