I heard a hadith claiming: "Whoever fasts for six days in Shawwal after Ramadan, is regarded as if he fasts the whole year." How reliable is this? As I heard Imam Abu Hanifah and Imam Malik consider this fast as makruh. I also want to ask: "Why do we acquire the merit of one year upon fasting for six days in Shawwal, could you please explain?"
Our response about the fast that is recommended to be performed in Shawwal is on the following link. Please read it for explanations:
About the second part of your question: “According to the considerations of scholars, the reason to be regarded as if fasted for one year by adding six more days of Shawwal to Ramadan fast is because the good deeds are recompensed ten times:
Ten times of Ramadan is ten months. Ten times the six days of Shawwal is sixty days, which are two months. And therefore, a person who fasts the Ramadan plus six days in Shawwal acquires the merit of a year (twelve months) fasting. ” (Ahmed Davudoğlu, Sahih Muslim Translation and Sharh, Sawm, 204 (1164.) explanation of hadith)
This explanations matches the verse in the Qur’an, because in the following verse the Glorified Allah commands:
“Whoever comes with a good deed will have ten times the like thereof, and whoever comes with an evil deed will not be recompensed except the like thereof; and they will not be wronged.” (Al-An’am/ The Cattle 6:160)