Mishkat al-Masabih 2179
‘Abdallāh b. Mas'ūd said, “Everything has a hump, and the hump of the Qur’ān is sūra al-Baqara. Everything has a kernel, and the kernel of the Qur’ān is al-Mufassal.*
*A title given to the sūras from 49 to the end, but several other sūras are also mentioned:
37, 45, 47, 48, 50, 61, 67, and 93. The name is most appropriately explained as meaning that this is the section of the Qur’ān which contains many shorter sūras . 

Dārimī transmitted it.
وَعَنْ عَبْدِ اللَّهِ بْنِ مَسْعُودٍ أَنَّهُ قَالَ: إِنَّ لِكُلِّ شَيْءٍ سَنَامًا وَإِنَّ سَنَامَ الْقُرْآنِ سُورَةُ الْبَقَرَةِ وَإِنَّ لِكُلِّ شَيْءٍ لُبَابًا وَإِنَّ لباب الْقُرْآن الْمفصل. رَوَاهُ الدَّارمِيّ