Superiority of Voluntary prayers
74) Narrated Abu Huraira that Allah's Messenger said: First of the deeds for which a slave will be called to account (on the Day of Resurrection) will be his (obligatory) prayers. If they are complete (he has prospered and succeeded), otherwise Allah will say (to the angels): See whether there are any (voluntary) prayers of My slave. If any, Allah will say: Complete his obligatory prayers with them.
(This Haditb is sound and reported by Ahmad in his Musnad, Abu Da'ud and Ibn Hibban).
On the Day of Resurrection, a believer willbe firstheld accountable for his obligatory prayers. If they are in order, he has succeeded. If they are wanting, the angels will be commanded to check his voluntary prayers, if any,theywill makefor theloss in his obligatory prayers. Thentherest of hisacts will bejudged.