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 ‏.‏