Sahih Muslim: Hadith 4600

Write" In the name of Allah, most Gracious and most Merciful." Suhail said: As for" Bismillah," we do not know what is meant by" Bismillah-ir-Rahman-ir-Rahim" (In the name of Allah most Gracious and most Merciful). But write what we understand, i. e. Bi ismika allahumma (in your name. O Allah). Then, the Prophet (ﷺ) said: Write:" From Muhammad, the Messenger of Allah." They said: If we knew that you welt the Messenger of Allah, we would follow you. Therefore, write your name and the name of your father. So the Prophet (ﷺ) said: Write" From Muhammad b. 'Abdullah." They laid the condition on the Prophet (ﷺ) that anyone who joined them from the Muslims, the Meccans would not return him, and anyone who joined you (the Muslims) from them, you would send him back to them. The Companions said: Messenger of Allah, should we write this? He said: Yes. One who goes away from us to join them-may Allah keep him away! and one who comes to join us from them (and is sent back) Allah will provide him relief and a way of escape.