Musannaf Ibn Abi Shaybah

Hazrat Amir considered that letter to be actionable which came to him sealed by the judge.

It is narrated from Hazrat Umar bin Abu Zaida that we brought a letter from the judge of Kufa to Hazrat Iyas bin Muawiyah. When I arrived, Hazrat Iyas had been removed from his position and Hasan had been appointed as the judge. I gave my letter to him, and he accepted it without asking me anything about it. Then he opened the letter and spread it out, and in it he found the testimony of two witnesses for me against a man of Basra regarding five hundred dirhams. Then he said to the person who was standing by him, "Take him to Ibn Ziyad and tell him to send him to so-and-so, and collect five hundred dirhams from him and give it to him." The narrator says that then he took me and did exactly as he had been told.

Ibrahim says that it is permissible for a judge to write a letter to another judge; it will be enforced.