In Tafsir Ruh al-Bayan, Hadrat Ismail Haqqi Bursawi states in brief:
The 160th verse of Surat al-An'am says (what means), "Whoever does a good deed will receive a tenfold reward. Whoever does an evil deed will be recompensed only with the like of it. None will be treated unjustly." This is a gift from Allahu ta'ala for Muslims. Non-Muslims do not earn rewards (thawab) for their good deeds because what is obligatory on them first is to believe. A good deed done without faith (iman) will not be beneficial in the Hereafter.
The deeds that are raghaib are not better than the deeds that are rawatib. [Nafila (voluntary) salats are divided into two: the ones that are rawatib and the ones that are raghaib. Rawatib ones are the sunnat salats performed before or after fard salats. Raghaib salats are the other nafila salats, such as Salat ad-Duha, Salat al-Awwabin, and Salat at-Tahajjud.] For example, a hadith-i sharif says, "Whoever performs 12 rak'ats of Salat ad-Duha, Allahu ta'ala will give him a mansion made of gold in Paradise." However, the pre-fard sunnat salat of Salat az-Zuhr is superior to Salat ad-Duha. Another hadith-i sharif says, "Whoever performs 6 rak'ats [Salat al-Awwabin] between Salat al-Maghrib and Salat al-Isha, Allahu ta'ala will record for him 12 years' acts of worship." However, the sunnat salat of Salat al-Maghrib is superior to Salat al-Awwabin. Such examples are many. The rewards for nafila acts of worship that are raghaib are not better than the rewards for the acts of worship that are rawatib, whose rewards were not explained.
Scholars declared unanimously that raghaib, rawatib, and wajib acts of worship cannot reach fard [obligatory] acts of worship in rewards, status, and superiority. Sunnat salats compensate for the deficiencies in one's fard salats. Nafila salats compensate for the deficiencies in one's sunnat salats. No nafila act of worship can acquit a person of a fard obligation. It has no place in Islam, unlike what the layman says, to claim that doing nafila acts of worship will substitute for obligatory acts of worship, such as claiming that performing Salat al-Awwabin will compensate for one's omitted [qada] salats. (Ruh al-Bayan, Vol. 3, p. 127)
In Ruh al-Bayan, the following hadith-i sharif is written in the tafsir of the 165th verse of Surat al-An'am:
On the Day of Resurrection, a person who earned money through unlawful (haram) means and who spent it on unlawful things will be brought to be questioned. It will be said, "Take him to Hell." Then someone else who earned money through lawful (halal) means and who spent it on lawful things will be brought to be questioned. It will be said to him:
"While you were earning or spending your wealth, you may have missed a salat that was obligatory on you or may have done ruku' and sajdas of it incompletely or may have omitted some of the obligatory elements of wudu'."
That person will say, "O my Rabb, I earned money through lawful means and spent it on lawful things. I never missed any obligatory act of worship."
It will be said to him again, "By your earnings, you acquired property, status, rank, and vehicle. However, you may have boasted of them and may have avoided giving them to the ones whom I ordered you to give, to your mahram relatives, to orphans, to poor people, and to stranded travelers?"
That person will say, "O my Rabb, I earned my wealth through lawful means and spent it on lawful things. I never missed any act of worship You made obligatory on me. I did not harm my ikhlas while I was doing obligatory acts of worship. I did not boast of my wealth. I gave everything completely that You ordered me to give."
Those who received help [zakat and the like] from this rich man will be brought to confront him, and they will say, "O my Rabb, among us, You gave wealth to him and ordered him to give [zakat and the like]. He, too, gave it to us without skipping any obligatory act and without any deceit." The questioning will continue this way. (Ruh al-Bayan, Vol. 3, p. 132)
Note that one is continuously being questioned about obligatory acts of worship. Allahu ta'ala said, "One cannot approach Me through anything as close as one approaches Me through fard acts of worship." His Messenger said, "The nafila salat of a person who has salats to make up will not be acceptable." Islamic scholars said, "Sunnat and nafila acts of worship compared with fard acts of worship are not even like a drop of water compared with an ocean." Based on these statements, is it not idiocy to spend time doing nafila acts of worship while one has fard salats to make up?