Muwatta Imam Malik (Riwayat Yahya)

Sa'id ibn al-Musayyib was asked about a person who made a vow to fast for a month. Is it permissible for him to observe voluntary fasts? He replied: He should first fulfill the vowed fasts, then observe voluntary fasts.

Imam Malik, may Allah have mercy on him, said: If a person dies and he had a vow upon him to free a slave, or to fast, or to give charity, or to offer a sacrifice, and then he makes a bequest that this vow should be fulfilled from his wealth, then it should be fulfilled from one third of the wealth, and its fulfillment should be considered prior to other bequests, except for those bequests which are obligatory like it. Because other bequests which are voluntary cannot be like this bequest, such as a vow, etc., therefore this is obligatory. And this bequest is specified to one third of the wealth because if it were to be executed from the entire wealth, then every person would delay such matters which are obligatory upon him until his death. When death approaches and the wealth becomes the right of his heirs, then at that time he would mention these things, especially those things for which there is no one to demand them, and perhaps these things would encompass all his wealth and the heirs would be deprived. Therefore, he does not have the right to do this from the entire wealth.

It reached Imam Malik, may Allah have mercy on him, that Abdullah bin Umar, may Allah be pleased with them both, was asked whether someone could fast on behalf of another person or pray on behalf of another person. He replied: No one should fast on behalf of another person, nor should anyone pray on behalf of another person.