It wasnarrated that Abu Qatadah said:
"Umar bin Khattab said: 'OMessenger of Allah! What about a person who fasts two days and doesnot fast one day?' He said: 'Is anyone able to do that?' Hesaid: 'OMessenger of Allah! What about a person who fasts one dayand not thenext?' He said: 'That is the fast of Dawud.' Hesaid: 'What about aman who fasts one day and does not fast thenext two days?' He said:'I wish that I were given the ability todo that.'"
English reference: Vol. 1, Book 7, Hadith 1713
Arabic reference: Book 7, Hadith 1784