Musnad Ahmad 937:221

Abu al-Yaman narrated to us, Safwan ibn ‘Amr narrated to us, from Shurayh ibn ‘Ubayd al-Hadrami, tracing it back to Abu Dharr, that he said: When the last ten days arrived, the Messenger of Allah (peace and blessings of Allah be upon him) performed I’tikaf in the mosque. When the Prophet (peace and blessings of Allah be upon him) prayed the ‘Asr prayer on the twenty-second day, he said: "We are standing [in prayer] tonight, if Allah wills, so whoever among you wishes to stand, let him stand." This was the night of the twenty-third. The Prophet (peace and blessings of Allah be upon him) prayed it in congregation after the night prayer (‘Atamah) until a third of the night had passed, then he departed. When it was the night of the twenty-fourth, he did not pray anything and did not stand. When it was the night of the twenty-fifth, he stood after the ‘Asr prayer on the twenty-fourth day and said: "We are standing tonight, if Allah wills," meaning the night of the twenty-fifth, "so whoever wishes, let him stand." He prayed with the people until a third of the night had passed, then he departed. When it was the night of the twenty-sixth, he did not say anything and did not stand. When it was at the time of the ‘Asr prayer on the twenty-sixth day, he stood and said: "We are standing, if Allah wills," meaning the night of the twenty-seventh, "so whoever wishes to stand, let him stand." Abu Dharr said: We braced ourselves for the standing, and the Prophet (peace and blessings of Allah be upon him) prayed with us until two-thirds of the night had passed, then he departed to his tent in the mosque. I said to him: "O Messenger of Allah, we had hoped that you would stand with us until the morning." He said: "O Abu Dharr, if you pray with your Imam and depart when he departs, it is written for you as the standing of your night."