From Sahih Muslim (Hadith 2741):
Abu Qatada reported that Abu al Malih informed me:I went along with your father to 'Abdullah b. Amr, and he narrated to us that the Messenger of Allah (ﷺ) was informed about my fasting and he came to me, and I placed a leather cushion filled with fibre of date-palms for him. He sat down upon the ground and there was that cushion between me and him, and he said to me: Does three days' fasting in a month not suffice you? I said: Messenger of Allah, (I am capable of observing more fasts). He said: (Would) five (not suffice for you)? I said: Messenger of Allah, (I am capable of observing more fasts) He said: (Would) seven (fasts) not suffice you? I said: Messenger of Allah, (I am capable of observing more fasts). He (the Holy Prophet) then said: (Would) nine (fasts not suffice you)? I said: Messenger of Allah, (I am capable of observing more fasts). He said: (Would) eleven (fasts not suffice you)? I said: Messenger of Allah, (I am capable of observing more fasts than these). Thereupon the Messenger of Allah (ﷺ) said: There is no fasting (better than) the fasting of David which comprises half of the age, fasting a day and not fasting a day
Reference: Muslim 2741