Majma al-Zawaid — Hadith 5064

Sayyiduna Awf bin Malik (may Allah be pleased with him) narrates: the Prophet (peace and blessings be upon him) was in i'tikaf during the last ten days of Ramadan. When the twenty-third night came, he said: "Whoever wishes to pray voluntary prayer with us tonight, let him do so." Then the Prophet (peace and blessings be upon him) led us in voluntary prayer until a third of the night had passed. Then he finished the prayer. I walked with him until he reached his tent. We submitted: "O Messenger of Allah! It would have been good if you had continued leading us in voluntary prayer this night." The Prophet (peace and blessings be upon him) said: "It is sufficient for a man to pray with the imam until the imam finishes the prayer (and then the man also finishes); it will be recorded for him as the voluntary prayer of a whole night."
Al-Tabarani recorded it in al-Mu'jam al-Kabir. Its chain contains Uthman bin Ata al-Khurasani, whom Duhaym declared thiqah (trustworthy), while the Imams declared him da'if (weak).