جامع الترمذي

Narrated Ibn 'Abbas: "I said to 'Uthman bin 'Affan: 'What was your reasoning with Al-Anfal - while it is from the Muthani (Surah with less than one-hundred Ayat), and Bara'ah while it is from the Mi'in (Surah with about one-hundred Ayat), then you put them together, without writing the line Bismillahir-Rahmanir-Rahim between them, and you placed them with the seven long (Surah) - why did you do that?' So 'Uthman said: 'A long time might pass upon the Messenger of Allah (ﷺ) without anything being revealed to him, and then sometimes a Surah with numerous (Ayat) might be revealed. So when something was revealed, he would call for someone who could write, and say: "Put these Ayat in the Surah which mentions this and that in it." When an Ayah was revealed, he would say: "Put this Ayah in the Surah which mentions this and that in it." Now Al-Anfal was among the first of those revealed in Al-Madinah, and Bara'ah among the last of those revealed of the Qur'an, and its narrations (those of Bara'ah) resembled its narrations (those of Al-Anfal), so we thought that it was part of it. Then the Messenger of Allah (ﷺ) died, and it was not made clear to us whether it was part of it. So it is for this reason that we put them together without writing Bismillahir-Rahmanir-Rahim between them, and we put that with the seven long (Surahs).'"