Sayyiduna Abu Musa al-Ash'ari (may Allah be pleased with him) narrates that the Prophet (peace and blessings be upon him) said: "I have been given the opening and closing words." We said: O Messenger of Allah, teach us some of the knowledge that Allah Almighty has given you (peace and blessings be upon him). So the Prophet (peace and blessings be upon him) taught us the words of the tashahhud.
Abu Ya'la recorded it; its chain contains a narrator, Abd al-Rahman bin Ishaq al-Wasiti, and he is da'if (weak).