Using the words 'May Allah protect you' in letters

Q 2: What is the ruling on the words “Long may you continue!” which is often written in letters or petitions, or “Accept my greetings,” “May Allah reward you with the best”, and “All the best”?

A: “Long may you continue” is said by way of Du‘a’ (supplication) for long life; but it is more proper to say, “May you continue in good.” May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.