All the virtuous deeds performed by a person may be divided into two categories: obligatory and voluntary. Obligatory deeds refer to that which is compulsory on man, such as the repayment of a debt or goodness in kind. In addition to these obligations, every good deed is accompanied by an aspect of voluntary service, that is to say, virtue which is above and beyond the call of duty. An example of this would be for one to reciprocate greater benevolence than that shown to him by another. This serves to complete and perfect compulsory actions. This Hadith expounds that the saints of Allah perfect their religious obligations through the observance of voluntary acts of worship. For example, in addition to Zakat, they also give charity. Allah the Exalted becomes the friend of such people and states that this relation grows to such intimacy that He becomes the hands, feet, etc. of such a person; in fact, He even becomes the tongue with which such a person speaks.
(Hazrat Mirza Ghulam Ahmadas, Malfuzat, Vol. 1, p. 12)