Beneficial knowledge is that which is received by the heart; bringing tranquility, fear, humility, and submission to Allah Ta’ala جل جلاله.
If this knowledge is not received by the heart, but rather by the tongue (The knowledge just remains on the tongue), then it is Allah Ta’ala’s جل جلاله proof against the son of Ādam, standing against its reciter and others (It will be a proof against the one who just merely states the knowledge with his tongue). As Hadhrat Ibn Mas’ood رضي الله عنه said: “There are people who recite the Qur’an, but it does not go beyond their collarbones, but when it enters the heart, it becomes deeply rooted and beneficial.”
Imam Hasan al Basri رحمه الله said: “Knowledge is of two types: knowledge of the tongue and knowledge of the heart. Knowledge of the heart is beneficial knowledge, and knowledge of the tongue is Allah Ta’ala’s جل جلاله proof against the son of Ādam.”
(Source: Al Khushoo’ fi Salaah)
Hafiz Ibn Qayyim al-Jawziyya رحمه الله