Mishkat al-Masabih 239
‘Abdallah b. ‘Amr reported God's messenger as saying, “Knowledge has three categories:
a precise verse, or an established sunna , or
a firm obligatory duty. Anything else is an extra.” Abu Dawud and Ibn Majah transmitted it.
وَعَنْ عَبْدِ اللَّهِ بْنِ عَمْرٍو قَالَ: قَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ: 
" الْعِلْمُ ثَلَاثَةٌ: آيَةٌ مُحْكَمَةٌ أَوْ سُنَّةٌ قَائِمَةٌ أَوْ فَرِيضَةٌ عَادِلَةٌ وَمَا كَانَ سِوَى ذَلِكَ فَهُوَ فضل ". رَوَاهُ أَبُو دَاوُد وَابْن مَاجَه