Mazhab/Madhab never mentioned in Quran but Deen has been mentioned.
Deen means a complete system, Social- Economic- Religious, whereas Mazhab only religious portion within the fold of Deen.
Quran Chapter 5 Surah Al Maidah verse 3:
Today I (Allah) have perfected your deen for you and completed my Blessing on you and approved Islam as your deen.
