B38 The Nature of Fasting

The Prophet (Sallahu Alayhi Wassallam) said:

The first thing that the servant will be brought to account for on the Day of Resurrection from his actions will be the Prayer. Our Lord will say to His angels, and He knows best, ‘Look at My Servant’s Prayer, did he complete it or was it deficient? If it is complete, then it will be recorded as complete. However, if there is any deficiency in it He will say, look and see whether my slave performed any supergatory Prayers: If he does have supergatory Prayers, He will say, ‘Complete the obligatory Prayers of My servant from his supergatory Prayers: Then the rest of his deeds will be judged in a similar fashion:’ Sahih Sunan Abu Dawud (no. 864)