Majma al-Zawaid — Hadith 3956

It is reported about Sayyiduna Mu'adh bin Jabal (may Allah be pleased with him) that one of his sons passed away, so the Prophet (peace and blessings be upon him) wrote him a letter offering condolences over his son. He wrote to him in the letter: "In the name of Allah, the Most Gracious, the Most Merciful. This is from Muhammad (peace and blessings be upon him), the Messenger of Allah, to Mu'adh bin Jabal. Peace be upon you. I praise before you Allah, besides whom there is no god. To proceed: May Allah Almighty grant you a great reward, grant you patience, and grant us and you the ability to be grateful. For our souls, our wealth and our families are gifts from Allah Almighty and things lent by Him for a time. Allah Almighty granted you joy and happiness through this child, and then took him back from you with abundant reward, in the form of blessings, mercy and guidance, if you hope for reward over him. So be patient, and let not your lamenting waste your reward, so that you later come to regret it. Know that wailing does not bring back the dead, nor does it remove grief. Whatever is to descend will surely come to pass. Peace be upon you."
Imam al-Tabarani recorded it in al-Mu'jam al-Kabir and al-Mu'jam al-Awsat. Its chain contains a narrator, Mujashi' bin Amr, and he is da'if (weak).