Musnad Ahmad 3:101

Isma'il ibn Ibrahim narrated to us, 'Awf ibn Abi Jamilah narrated to us, Yazid al-Farisi narrated to me, from Ibn 'Abbas, who said: I said to 'Uthman: "What compelled you to take Al-Anfal, which is of the Mathani, and Bara'ah, which is of the Mi'in, and join them together, and not write between them the line 'In the name of Allah, the Entirely Merciful, the Especially Merciful,' and place them among the Seven Long [Surahs]? What compelled you to do that?" He said: "It used to be that when time passed over the Messenger of Allah (peace and blessings of Allah be upon him), and Surahs of many verses were revealed to him, whenever something was revealed to him, he would call some of those who used to write for him and say: 'Place these in the Surah in which such and such is mentioned.' And when verses were revealed to him, he would say: 'Place these verses in the Surah in which such and such is mentioned.' And when a verse was revealed to him, he would say: 'Place this verse in the Surah in which such and such is mentioned.' Al-Anfal was among the first of what was revealed in Al-Madinah, and Bara'ah was among the last of what was revealed of the Qur'an. Its story was similar to its story, so we thought that it was part of it. The Messenger of Allah (peace and blessings of Allah be upon him) was taken [in death] and he had not clarified to us that it was part of it. For that reason, I joined them together, did not write between them the line 'In the name of Allah, the Entirely Merciful, the Especially Merciful,' and placed them among the Seven Long [Surahs]."