Musannaf Ibn Abi Shaybah 18971 — A man writing his wife's divorce

Hammad (may Allah have mercy on him) said: If a man wrote a letter to his wife and wrote in it, "When this letter of mine reaches you, you are divorced," then if the letter does not reach her, no divorce takes place. And if he wrote in the letter, "Amma ba'd (to proceed): you are divorced," then divorce takes place even if the letter does not reach her.
Ibn Shubrumah said: She is divorced.