Musnad Ahmad 288:11

'Abd al-Rahman ibn Mahdi narrated to us, 'Abd Allah ibn 'Abd al-Rahman al-Ta'ifi narrated to us, from 'Uthman ibn 'Abd Allah ibn Aws al-Thaqafi, from his grandfather Aws ibn Hudhayfah, who said: I was among the delegation that came to the Prophet, may Allah bless him and grant him peace, having accepted Islam from Thaqif, from the tribe of Banu Malik. He lodged us in a tent of his, and he would go back and forth to us between his houses and the mosque. When he prayed the final 'Isha' prayer, he would turn to us, and we would not depart until he spoke to us and complained about Quraysh and complained about the people of Makkah. Then he would say: "It is not the same. We were in Makkah, humiliated and oppressed. When we went out to Madinah, the buckets of war were between us and them." He stayed away from us one night and did not come to us until it became long for us after 'Isha'. We said: "What kept you from us, O Messenger of Allah?" He said: "A portion (hizb) of the Qur'an occurred to me, and I did not want to go out until I had completed it." He said: So we asked the companions of the Messenger of Allah, may Allah bless him and grant him peace, when it became morning. We said: "How do you divide the Qur'an into portions?" They said: "We divide it into three surahs, five surahs, seven surahs, nine surahs, eleven surahs, thirteen surahs, and the portion of the Mufassal from Qaf until the end."