until he said to him: “Fast one day and do not fast the next; that is the best fast, and it was the fast of David.” Regarding the night prayer, he said to him: “Sleep for half the night, stand in prayer for a third of it, then sleep for a sixth of it. That is the best manner of standing in prayer, and it was David’s manner of standing in prayer.” (1) This is because such fasting gives the soul a degree of freedom and the body a degree of strength, since he fasts one day and does not fast the next. Likewise, if one sleeps for half the night, then stands for a third of it, and then sleeps for a sixth of it, the fatigue from standing for that third will be relieved by his sleeping for the sixth, so that he begins the day invigorated.
However, this—fasting one day and not fasting the next—is conditional upon its not causing him to neglect what Allah has made obligatory upon him. If it causes him to neglect what Allah has made obligatory, then it is prohibited, for an obligatory duty cannot be neglected for the sake of a voluntary act. Thus, if it were the case that, when this man fasted one day and did not fast the next, he missed the congregational prayer in the mosque because he became tired at the end of the day and was unable to reach the mosque, we would say to him: Do not do this. Neglecting an obligation is more serious than neglecting a recommended act. This is merely recommended; you incur no sin by leaving it, so leave it.
Likewise, if this occupied him to the neglect of providing for his family—that is, if it caused him to cease buying, selling, and working as needed to support his family—we would say to him: Do not do this, for fulfilling an obligation is more important than performing a voluntary act. Similarly, if this fasting resulted in his failing to fulfill the duties of his employment, it would be prohibited.
Abdullah ibn Amr, may Allah be pleased with them both, remained committed to this until he grew old, and then wished that he had accepted the Prophet’s concession, peace and blessings be upon him, to fast three days of every month. Yet he strove hard, may Allah have mercy on him, and so he would fast fifteen consecutive days