Engage yourself all the time in the dhikr of God. Remember that everything you do according to the Shar‘ is dhikr even if it is so ordinary an act as buying and selling. Observe, therefore, the rules of the Shar‘ in all activities so that the whole of life becomes dhikr. In fact dhikr means to avoid forgetting; so when you obey the ‘dos’ and ‘don’ts’ of the Shar‘ in all your behaviour, you do not forget the Giver of the Shar‘ and remember Him perpetually.
This perpetual remembrance (dhikr dā’im) is different from the perpetual awareness (yād dāsht) of our (Naqshbandī) masters (may God bless their souls). The latter is only a matter of the heart (bāṭin), whereas the former embraces the inner heart as well as the outer behaviour; hence it is difficult. May God help us to follow the way of the Prophet, and shower His blessings upon him!
[Maktubāt, Vol. II:25. English translation by Muhammad Abdul Haq Ansari in Sufism and Shari‘ah, Leicester: Islamic Foundation, 1986 p. 233]