six (6) Rozas of Shawwal [Nafils (surplus) Fasting during the month of Shawwal (Shawwaal, Shawal)]

Hazrat Abu Ayyoob Ansaari (r.a) reported that the Holy Prophet (Peace be upon him) said that the one who will have observed roza (fasting) during the entire Holy month of Ramdan (Ramzan, Ramadwaan) and afterwards observe six (6) days of roza (fasting) nafils (surplus) during the month of Shawwal (Shawwaal, Shawal), will be considered as someone who will have fasted during one whole year.

[Reference. Muslim, Abu-Daawood, Tirmizi, Ibn Maajaa, Mishkaat]