Al-Mu'jam al-Awsat 83 — a son of his died, and the Messenger of Allah

It is narrated from Sayyiduna Mu'adh bin Jabal (may Allah be pleased with him) that a son of his died, and the Messenger of Allah (peace and blessings be upon him) wrote him a letter of condolence on the death of his son, in which he (peace and blessings be upon him) wrote: "In the name of Allah, the Most Gracious, the Most Merciful. From Muhammad, the Messenger of Allah (peace and blessings be upon him), to Sayyiduna Mu'adh bin Jabal (may Allah be pleased with him). Peace be upon you. I praise before you Allah, besides whom there is none worthy of worship. To proceed: May Allah Almighty make your reward great, inspire you with patience, and bless us and you with gratitude. Indeed our souls, our wealth, our families and our children are among the pleasant gifts of Allah, the Mighty and Majestic, and His loans entrusted (to us). Allah let you enjoy him with joy and happiness, and now, taking him back from you, He has promised in return a great reward and blessing (forgiveness), mercy and guidance. If you hope for reward from Allah, then be patient, and let not your impatience destroy your reward so that you come to regret it. Know that impatience neither brings back the dead nor removes grief, and what is to come will surely come. And peace be upon you."
This hadith is narrated from Sayyiduna Mu'adh (may Allah be pleased with him) only with this chain, and Mujashi' (may Allah have mercy on him) is alone in narrating it.