Musannaf Ibn Abi Shaybah

Hazrat Hammad (may Allah have mercy on him) says that if a man wrote a letter to his wife and wrote in it that when this letter reaches you, you are divorced, then if the letter does not reach her, the divorce will not take place. And if he wrote in the letter: "Amma ba'd! You are divorced," then even if the letter does not reach her, the divorce will take place.