Sahih Muslim Book 006, Hadith Number 2501.

Chapter : Fasting on the day of 'Ashura (1Oth of Muharram).

'Aisha (Allah be pleased with her) reported. In the Pre-Islamic days fast was observed on the day of Ashura, but with the advent of Islam (its position was ascertained as that of a voluntary fast). Then he who wished to fast fasted, and he who liked to abandon it abandoned it.

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