Musnad Ahmad 830:110

Rawh and Abu Dawud narrated to us, they said: Hammad ibn Salamah narrated to us. Abu Dawud said: Ali ibn Zayd narrated to us, from al-Hasan, from Abu Bakrah, who said: "The Messenger of Allah (peace and blessings of Allah be upon him) delayed the Isha prayer for nine nights—Abu Dawud said: eight nights—until one-third of the night had passed." Abu Bakr said: "O Messenger of Allah, if you had hastened it, it would have been better for our standing in the night." He said: "So he hastened it after that." Abd al-Samad narrated to us, and he said in his hadith: "nine nights," and Affan said: "seven nights."