Abu Wa'il (may Allah have mercy on him) said: A letter from Sayyiduna Umar bin al-Khattab (may Allah be pleased with him) came to us, (saying): When you besiege a fortress and they want you to bring them down on the judgement of Allah, do not do so, for you do not know what the judgement of Allah regarding them is; rather, bring them down on your own judgement, then decide regarding them as you wish. And when a man says to another man: 'Do not fear,' he has given him safety; and when he says the word 'matras,' he has given safety; and when he says the words 'la tadhal' (do not be alarmed), he has also given protection, for Allah Almighty knows the languages.
Al-Thawri (may Allah have mercy on him) narrates it from al-A'mash (may Allah have mercy on him), and at the end of it is: Protection is also given with the words 'la tadhhal' (do not be alarmed), for Allah, the Lord of Might, knows the languages.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب السير / 18180