This excerpt is taken from Sheikh Abu Hamza Abdul Khaliq Siddiqi's book Sahi Fazail-e-Amaal.
عن أبى أيوب رضي الله عنه، أن رسول الله صلى الله عليه وسلم قال: من صام رمضان، ثم أتبعه ستا من شوال، كان كصيام الدهر
It is narrated from Sayyiduna Abu Ayyub رضي الله عنه that the Messenger of Allah صلی اللہ علیہ وسلم said: "Whoever fasts Ramadan and then follows it with six (voluntary) fasts in Shawwal, it is as if he fasted for the entire year."
Reference: Sahih Muslim, Book of Fasting, Chapter on the recommendation of fasting six days of Shawwal following Ramadan, No: 1164.
عن أبى قتادة رضى الله عنه قال: سئل رسول الله صلى الله عليه وسلم عن صوم يوم عرفة؟ قال: يكفر السنة الماضية والباقية .
Sayyiduna Abu Qatadah رضي الله عنه reports that the Messenger of Allah صلی اللہ علیہ وسلم was asked about the fast of Arafah, and he صلی اللہ علیہ وسلم said: "It expiates the sins of the previous year and the coming year."
Reference: Sahih Muslim, Book of Fasting, Chapter on the recommendation of fasting three days of every month and fasting on the day of Arafah, No: 1162