Roza nafil in Shawwal
Hazrat Abu Ayyoob Ansaari (r.a) reported that the holy prophet ( peace be upon him ) said that the one who will have observed roza during the entire holy month of Ramadwaan and afterwards observe six (6) days of roza nafils (surplus) during the month of Shawwaal, will be considered as someone who will have fasted during one whole year. [ref. Muslim, Abu-Daawood, Tirmizi, Ibn Maajaa, Mishkaat]
Now we shall consider an arithmetic fact on that hadith. As we know, it is said that when one make a good deed, one obtain in return ten (10) times more rewards. In the case that there is (a maximum of) 30 days of rozas in the month of Ramadwaan, which means that we obtain the thawaab (reward) equivalent to 30 x 10 = 300 days of roza.
However, in a lunar year, it can bear a maximum of 360 days. The fact to observe 6 more days as nafil, bring the sum to 6 x 10 = 60. So, logically 300 + 60 = 360, as stated by the wisdom of the holy prophet ( peace be upon him ) in this hadith, that is if somebody put this recommendation into practice will get the rewards of one whole year of rozas.
Hazrat ‘Abdullah Ibn Umar (r.a) reported that Rasoolullaah said « The one who observe the rozas of the month of Ramadwaan entirely and the six nafils of the month of Shawwaal, will have his sins erased as if he just took birth ». [Tibrani]
However, let us precise that the six rozas nafil can be observed consecutively or on inervals (seperately) during the month of Shawwaal, but necessarily before its end.