Sunan An-Nasa'i

It was narrated that 'Abdullah bin 'Amr said:'Amr said: ""I spoke to the Prophet and he said: 'Fast one day out of ten and you will have the reward of the other nine.' I said: 'I am able to do more than that.' He said: 'Fast one day out of eight and you will have the reward of the other eight.' I said: 'I am able to do more than that.' He said: 'Fast one day out of eight and you will have the reward of the other seven.' I said: 'I am able to do more than that.' 'Fast one day and not the next