Sahih (Sahih Muslim)
وَحَدَّثَنَا نَصْرُ بْنُ عَلِيٍّ الْجَهْضَمِيُّ، حَدَّثَنِي أَبِي، حَدَّثَنَا لَيْثُ بْنُ سَعْدٍ، بِهَذَا الإِسْنَادِ . بِمِثْلِهِ غَيْرَ أَنَّهُ قَالَ " فَإِنَّ فِي السَّنَةِ يَوْمًا يَنْزِلُ فِيهِ وَبَاءٌ " . وَزَادَ فِي آخِرِ الْحَدِيثِ قَالَ اللَّيْثُ فَالأَعَاجِمُ عِنْدَنَا يَتَّقُونَ ذَلِكَ فِي كَانُونَ الأَوَّلِ .
This hadith is reported on the authority of Laith b. Sa'd with the same chain of transmitters, but with a slight variation in wording (and that is that) he (the Holy Prophet) said:There is a day in a year when descends the pestilence; at the end of the hadith Laith said that the non Arabs save themselves from it in Kanun Awwal (this is the month of December)