Is it permissible to combine the fasting of the Sunnah of the Prophet Muhammad, may God bless him and grant him peace, with the fasting of the Prophet David, peace be upon him, or does the fasting of David suffice for voluntary fasting?

The fast of David (peace be upon him) is the best of fasts and the most beloved to Allah Almighty. It is not recommended to exceed it, because the Prophet (peace and blessings of Allah be upon him) said: "There is nothing better than that." This includes additions that permanently alter the form of David's fast, such as fasting on Mondays and Thursdays and the White Days. As for fasting on rare days like 'Arafah and 'Ashura, there is no harm in it, and it does not remove one from the circle of preferability, because these are rare and do not affect the form of David's fast.