There are two types of knowledge: knowledge of the tongue, which can be a case against the son of Adam, as is mentioned in the hadith of the Prophet (S): ‘The Qur’an is either a case for you or a case against you’; and knowledge of the heart, which is beneficial knowledge. The second type is the beneficial kind which raises people in rank; it is the inner knowledge which is absorbed by the heart and puts it right. The knowledge that is on the tongue is taken lightly by people: neither those who possess it, nor anyone else, act upon it, and then it vanishes when its owners vanish on the Day of Judgement, when creation will be brought to account.
—Al-Ḥasan al-Baṣrī [d 110H/728CE]
(Read on pg 13, Ibn Rajab, Ibn al-Qayyim, al-Ghazali, The Purification of the Soul. Al-Firdous Ltd. London:1993.)