Sahih Bukhari
Prophetic Commentary on the Quran (Tafseer of the Prophet (PBUH))
Volume: 006 / Book Number: 060 / Hadith Number: 261
Narrated Ibn Abbas:
When Allah's Apostle arrived at Medina, he found the Jews observing the fast on the day of 'Ashura' (10th of Muharram). The Prophet asked them (about it) and they replied, "This is the day when Moses became victorious over Pharaoh." The Prophet said (to the Muslims), "We are nearer to Moses than they, so fast on this day."