Abu al-Hasan ‘Ali reported: Al-Junayd ibn Muhammad, may Allah have mercy on him, said, “Our knowledge of Tasawwuf is defined by the Book and the Sunnah. Whoever does not memorize the Quran, nor write the prophetic teachings, nor study their understanding, he is not to be followed in it.”
Source: Ḥilyat al-Awliyā’ 10/255
عن أبي الْحَسَنِ عَلِيَّ قال قال الْجُنَيْد بْن مُحَمَّدٍ رحمه الله عِلْمُنَا يعني ٱلتَّصَوُّف مَضْبُوطُ الْكِتَابِ وَالسُّنَّةِ مَنْ لَمْ يَحْفَظِ الْقُرْآنَ وَلَمْ يَكْتُبِ الْحَدِيثَ وَلَمْ يَتَفَقَّهْ لَا يُقْتَدَى بِهِ
10/255 حلية الأولياء وطبقات الأصفياء
