It is narrated from Sayyiduna Muslim bin al-Harith bin Badal al-Tamimi (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) wrote for him a letter of instruction addressed to those whom Allah would grant authority after him, and sealed it.
Hadith Referenceالمعجم الكبير للطبراني / باب الميم / 17102