The person who fasts and performs all other obligatory actions is from the best servants of Allah. And those who are negligent in fulfilling their obligations and do not give Allah His full due, then woe be to them. If the individual who deals unjustly with the scales (Mutaffif) in this worldly life is warned of a terrible punishment, then what is the case with the individual who deals unjustly with the scales of the hereafter!
The Salaf worked diligently in perfecting their actions and making them faultless, then after that, they showed great concern for acceptance of their actions. They did this because they feared that their actions would be rejected. They are those who gave what was incumbent upon them while their hearts were in a state of fear. It is reported that Ali bin Abi Taleb said: Be more concerned that your actions are accepted than your concern of performing the action itself. Did you not hear the statement of Allah:
Verily Allah only accepts the actions of the Muttaqeen (pious) – Surah Al-Ma’idah [5:27]
The above are excerpts from Ibn Rajabýs book: Lataif al Maaýrif.