Musnad Ahmad 399
It was narrated that Yazeed said:
Ibn ‘Abbas (رضي الله عنه) said to us.  I said to ‘Uthman bin `Affan: What made you take al-Anfal, which is one of al-Mathani (the seven long soorahs), and Bara`ah, which is one of al-Mi`een (soorahs with one hundred verses or thereabouts), and put them next to one another and not write - Ibn Ja`far said: A line between them saying Bismillahir-Rahmanir-Raheem - and you put them with the seven long ones? What made you do that? `Uthman (رضي الله عنه)said: Sometimes many soorahs would be revealed (incomplete) to the Messenger of Allah (ﷺ) , and when something was revealed he would call one of the scribes to write it down for him and say: “Put this in the Soorah in which such and such is mentioned`; and verses would be revealed to him and he would say, `Put these verses in the soorah in which such and such is mentioned”; and a verse would be revealed to him and he would say: `Put this verse in the soorah in which such and such is mentioned.” Al-Anfal was one of the first soorahs to be revealed in Madinah and Bara`ah was one of the last soorahs of the Qur`an, and the stories and content of the two soorahs were similar. 
Then the Messenger of Allah (ﷺ)  passed away without having stated clearly to us that it was part of it, but we thought that it was, hence I put them together and I did not put between them the line Bismillahir-Rahmanir-Raheem. Ibn Ja`far said: I put it with the seven long ones.
حَدَّثَنَا يَحْيَى بْنُ سَعِيدٍ، حَدَّثَنَا عَوْفٌ، حَدَّثَنَا يَزِيدُ يَعْنِي الْفَارِسِيَّ، قَالَ أَبِي أَحْمَدُ بْنُ حَنْبَلٍ و حَدَّثَنَا مُحَمَّدُ بْنُ جَعْفَرٍ، حَدَّثَنَا عَوْفٌ، عَنْ يَزِيدَ، قَالَ قَالَ لَنَا ابْنُ عَبَّاسٍ رَضِيَ اللَّهُ عَنْهُ قُلْتُ لِعُثْمَانَ بْنِ عَفَّانَ مَا حَمَلَكُمْ عَلَى أَنْ عَمَدْتُمْ، إِلَى الْأَنْفَالِ وَهِيَ مِنْ الْمَثَانِي وَإِلَى بَرَاءَةٌ وَهِيَ مِنْ الْمِئِينَ فَقَرَنْتُمْ بَيْنَهُمَا وَلَمْ تَكْتُبُوا قَالَ ابْنُ جَعْفَرٍ بَيْنَهُمَا سَطْرًا بِسْمِ اللَّهِ الرَّحْمَنِ الرَّحِيمِ وَوَضَعْتُمُوهَا فِي السَّبْعِ الطِّوَالِ مَا حَمَلَكُمْ عَلَى ذَلِكَ قَالَ عُثْمَانُ رَضِيَ اللَّهُ عَنْهُ إِنَّ رَسُولَ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ كَانَ مِمَّا يَأْتِي عَلَيْهِ الزَّمَانُ يُنْزَلُ عَلَيْهِ مِنْ السُّوَرِ ذَوَاتِ الْعَدَدِ وَكَانَ إِذَا أُنْزِلَ عَلَيْهِ الشَّيْءُ يَدْعُو بَعْضَ مَنْ يَكْتُبُ عِنْدَهُ يَقُولُ ضَعُوا هَذَا فِي السُّورَةِ الَّتِي يُذْكَرُ فِيهَا كَذَا وَكَذَا وَيُنْزَلُ عَلَيْهِ الْآيَاتُ فَيَقُولُ ضَعُوا هَذِهِ الْآيَاتِ فِي السُّورَةِ الَّتِي يُذْكَرُ فِيهَا كَذَا وَكَذَا وَيُنْزَلُ عَلَيْهِ الْآيَةُ فَيَقُولُ ضَعُوا هَذِهِ الْآيَةَ فِي السُّورَةِ الَّتِي يُذْكَرُ فِيهَا كَذَا وَكَذَا وَكَانَتْ الْأَنْفَالُ مِنْ أَوَائِلِ مَا أُنْزِلَ بِالْمَدِينَةِ وَبَرَاءَةٌ مِنْ آخِرِ الْقُرْآنِ فَكَانَتْ قِصَّتُهَا شَبِيهًا بِقِصَّتِهَا فَقُبِضَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ وَلَمْ يُبَيِّنْ لَنَا أَنَّهَا مِنْهَا وَظَنَنْتُ أَنَّهَا مِنْهَا فَمِنْ ثَمَّ قَرَنْتُ بَيْنَهُمَا وَلَمْ أَكْتُبْ بَيْنَهُمَا سَطْرًا بِسْمِ اللَّهِ الرَّحْمَنِ الرَّحِيمِ قَالَ ابْنُ جَعْفَرٍ وَوَضَعْتُهَا فِ السَّبْعِ الطِّوَالِ‏.‏