As for the believer, it is essential that after his prayer is answered tht he worships Allah, and his sincere to Him, and turns to Him as he has been commanded. This can be achieved either by doing only the obligatory acts —and in this case he is among the pious — or by doing the obligatory and encourage acts — and in this case he will be among those who are close to Allah.  And whoever leaves part of what he has been commanded with after his prayer has been answered, then he is among the sinners.  And this in fact might enter into minor shirk, which most of the creation is tested with. Or, it might enter into shirk in Allah’s Oneness (Ruboobiyyah), or shirk in Allah’s right to be worshiped alone (Uloohiyyah).

—Ibn Taymiyyah [d. 728H/1328CE]

Qa’idah fi Anwa’al-Istiftah, p. 17

(Read on pg 223, Yasir Qadhi, Du’a: The Weapon of the Believer. Al-Hidaayah Publishing & Distribution Ltd. Birmingham: 2003.)