Musnad Ahmad 937:265

Muhammad ibn Ja‘far narrated to us, ‘Awf narrated to us, from Muhajir Abu Khalid, Abu al-‘Aliyah narrated to me, Abu Muslim narrated to me, he said: I said to Abu Dharr: Which standing of the night is best? Abu Dharr said: I asked the Messenger of Allah, may Allah bless him and grant him peace, just as you asked me—‘Awf was uncertain—so he said: "The middle of the latter part of the night, or half of the night, and few are those who do it."