ما المقصود بالعمل الذي يجب أن يقترن به كل من الدعاء والذكر؟

The intention behind the necessity for du'a (supplication) and dhikr (remembrance of God) to be accompanied by the heart's action is the obligation of the heart's presence and its awareness of what the tongue is uttering. This is not a shar'i (legal) obligation for which one is sinful if he abandons it. Indeed, heedlessness concerning the contemplation of du'a may prevent its effect. Furthermore, the accompanying of du'a and dhikr with righteous deeds prevents deeds from becoming nullified and from not being rewarded. Ibn al-Qayyim mentioned that the effect of du'a might be withheld due to the weakness of the du'a itself, or due to the weakness of the heart and its lack of turning to God, or due to the presence of impediments such as consuming the unlawful, injustice, and the stain of sins. The proof for this is the saying of the Prophet, peace be upon him: "Supplicate to Allah while being certain of the response, and know that Allah does not accept the supplication from a heedless, inattentive heart." And consuming the unlawful invalidates the power of du'a, as stated in the hadith: "A man prolongs his journey, disheveled and dusty, extending his hands to the sky, saying, 'O Lord, O Lord!' while his food is unlawful, his drink is unlawful, his clothing is unlawful, and he is nourished with the unlawful. How can he be answered then?" Nevertheless, du'a and dhikr without the heart's presence are better than abandoning them altogether.