What is the best manner of voluntary fasting one can undertake?

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The best voluntary fasting is to fast one day and break the fast the next day. This practice is known as the fast of Dawud. Abdullah ibn Amr reported that the Prophet, peace be upon him, instructed him to fast a day and break a day, stating it was the best fast. When Abdullah offered to do more, the Prophet responded, 'There is no better fast than that.' This ruling is established by agreement (Muttafaqun 'alayh).