Musnad Ahmad 265:5

Ya'qub narrated to us, he said: My father narrated to us, from Ibn Ishaq, he said: Mu'adh ibn 'Abdullah ibn Khubayb al-Juhani narrated to me, from his brother 'Abdullah ibn 'Abdullah ibn Khubayb, he said: There was a man in the time of 'Umar ibn al-Khattab who had asked him, so he gave him. He said: 'Abdullah ibn Unays, the companion of the Messenger of Allah, may Allah bless him and grant him peace, sat with us in his gathering—in the gathering of Juhaynah—he said: In Ramadan. He said: We said to him: "O Abu Yahya, did you hear anything from the Messenger of Allah, may Allah bless him and grant him peace, regarding this blessed night?" He said: "Yes, we sat with the Messenger of Allah, may Allah bless him and grant him peace, at the end of this month, and we said to him: 'O Messenger of Allah, when should we seek this blessed night?'" He said: "Seek it this night." And he said: That was the evening of the twenty-third night. A man from the people said to him: "Is it then, O Messenger of Allah, the beginning of the eight?" He said: The Messenger of Allah, may Allah bless him and grant him peace, said: "It is not the beginning of the eight, but it is the beginning of the seven, for the month does not complete."