Sunan Ibn Majah — The Book of Purification and its Sunnah

They all heard the Messenger of Allah say: ‘Complete the ablution. Woe to the heels because of Hell-fire.”

“Umar bin Khattab said: ‘O Messenger of Allah! What about a person who fasts two days and does not fast one day?’ He said: ‘Is anyone able to do that?’ He said: ‘O Messenger of Allah! What about a person who fasts one day and not the next?’ He said: ‘That is the fast of Dawud.’ He said: ‘What about a man who fasts one day and does not fast the next two days?’ He said: ‘I wish that I were given the ability to do that.’”