It is narrated from Sayyiduna Amr bin Aws al-Thaqafi (may Allah be pleased with him); he said: I came to the Messenger of Allah (peace and blessings be upon him) with the delegation of Thaqif. He (peace and blessings be upon him) used to come out to us at night and talk with us. One night he (peace and blessings be upon him) was late, then he (peace and blessings be upon him) came out to us, and we said: O Messenger of Allah, you were late tonight. He (peace and blessings be upon him) said: "My (daily appointed) portion of the Qur'an came upon me, and I disliked leaving it before completing it." When morning came, we asked the Companions of the Messenger of Allah (peace and blessings be upon him) how they divided the Qur'an into portions. They said: Three, five, seven, nine, eleven and thirteen (surahs to a portion), and from "Qaf. By the glorious Qur'an" (Surah Qaf) to the end of the Mufassal is one fine portion.
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 14159