Tasawwuf is Farz
Acquiring the knowledge that the Sufis call "Laduni" (knowledge directly from Allah) is Farz-e-Ain (compulsory obligation), because the fruit of this knowledge is the purification of the Qalb from being engaged in non-Allah, and the Qalb's perpetual engagement in Zikr. Its result is the cleansing of the Nafs from immoral behavior such as vanity, envy, love of the world, love of power, slackness in worships, following carnal desires, pretentious behavior, etc.And its outcome is the inculcation of high morals such as asking Allah for forgiveness for one's sins, being contented with Allah's Will, being thankful for Allah's bounties, and being patient/steadfast in distress, etc.
There is no doubt that the above-mentioned sins are even more strictly forbidden for the believe than the sins of limbs and senses. And (the above-mentioned merits) are more important obligations (Farz) than prayer, fasting, and Zakat. Because worship is useless without sincerity, and Tasawwuf is just another name for sincerity (Ikhlas).
Hazrat Qazi Sanaullah Panipatti, Tafseer-e-Mazhari, via Dalail-us-Salook