Hadithcore
Sahih Muslim, 1080 o
sahih

This hadith has been narrated on the authority of Aswad b. Qais with the same chain of transmitters, but herein no mention has been made of the other month (consisting of) thirty days.

وَحَدَّثَنِيهِ مُحَمَّدُ بْنُ حَاتِمٍ، حَدَّثَنَا ابْنُ مَهْدِيٍّ، عَنْ سُفْيَانَ، عَنِ الأَسْوَدِ بْنِ قَيْسٍ، بِهَذَا الإِسْنَادِ وَلَمْ يَذْكُرْ لِلشَّهْرِ الثَّانِي ثَلاَثِينَ ‏.‏

References2 variants
In-Book Reference
Book 13, Hadith 18
USC-MSA web (English) reference
Book 6, Hadith 2376 (deprecated numbering scheme)