كتاب الصيام
13  
The Book of Fasting
(27)
Chapter: Making up fasts on behalf of the deceased
(27)
باب قَضَاءِ الصِّيَامِ عَنِ الْمَيِّتِ
Sahih Muslim 1149 d

This hadith has been narrated on the authority of Sufyan with the same chain of transmitters in which it is said:

" Fasting of two months."
وَحَدَّثَنِيهِ إِسْحَاقُ بْنُ مَنْصُورٍ، أَخْبَرَنَا عُبَيْدُ اللَّهِ بْنُ مُوسَى، عَنْ سُفْيَانَ، بِهَذَا الإِسْنَادِ وَقَالَ صَوْمُ شَهْرَيْنِ ‏.‏
Reference : Sahih Muslim 1149d
In-book reference : Book 13, Hadith 206
USC-MSA web (English) reference : Book 6, Hadith 2560
  (deprecated numbering scheme)