The ninth indication regarding the esoteric meaning of the supererogatory prayers.

The Ninth Indication: On the Inward of the Supererogatory Prayers [Nawafil]

Know, O righteous brother—may Allah the Exalted grant you success—that the Supererogatory prayers in the outward are not obligatory according to Allah, but rather recommended. However, among the people of the inward, they are mandatory, and the difference between the recommended (mustahabb), the mandatory (wajib), and the obligatory (fard) is the Muslim's increase in performing obedience beyond the established capacity [enacted] by the Messenger. The mandatory is performing what Allah commanded to safeguard of the legislated acts of worship for the people of the revealed Books. The ruling of the recommended in the outward and the inward necessitates for the Gnostic [the risk of] hesitation and punishment through dark veils; whereas rejecting the obligatory necessitates eternity in the blazing fire because it is disbelief. No Gnostic shall enter the highest kingdom without performing all the mandatory obligatory acts, because the supererogatory (nafila) is in reality legislated for the Gnostic [regardless of his standing] or otherwise. To this, the indication occurred in His saying: {And from the night, pray with it as a supererogatory for you; perhaps your Lord will resurrect you to a praised station} (Al-Isra 79).