The acts which bring us close to God are obligatory (farḍ) as well as supererogatory (nafl). But (in the order of merit) the latter stand no comparison to the former. To perform a farḍ at one time is better than performing a nafl act for a thousand years, even if it is done with an absolutely pure motive, no matter whether it is prayer, charity, fasting, dhikr, meditation (fikr) and the like. To engage in a non-obligatory act hallowed by the practice (sunnat) of the Prophet [صلى الله عليه وسلم], or to observe a rule of decency or morality (adab) at the time of a duty (farḍ) is in the same category.
It has been reported that one day Amir al-Mu’minīn ‘Umar offered the morning prayer in assembly, turned to the people, and did not find a certain person. He enquired as to what would have possibly detained him from praying in assembly. They said that he was in the habit of waking up at night to offer prayers; perhaps he might have slept on after night prayers and could not wake up in time. Thereupon, ‘Umar said: ‘If he had slept the whole night and joined the morning prayer in assembly that would have been far better for him.’
Hence to obey an adab and avoid that which is undesirable (makrūh) in some degree, let alone that which is prohibited, is better than remembrance, contemplation and meditation. If one can do these things along with observing rules and avoiding the undesirables, that would certainly be a great achievement, otherwise it would be a great loss. To spend, for instance, a penny of zakāt is many times better than giving mountains of gold in supererogatory charity; similary, to mind an adab of the Shar‘ in spending that penny, such as to give to it to a poor relative, is better than spending it without minding that adab.
[Maktubāt, Vol. I:29. English translation by Muhammad Abdul Haq Ansari in Sufism and Shari‘ah, Leicester: Islamic Foundation, 1986 p. 234]