[ Muslim ]
Ibn Abbas (RadhiAllahu 'anhu) reported : The Messenger of Allah (salAllahu alayhi wa sallam) said, “If I remain alive till the next year, I shall also observe fast on the ninth of Muharram."
[ Muslim ]
Commentary:
Some people think these words stand to mean that `he will observe Saum on the 9th of Muharram only. But this interpretation does not agree with other Ahadith. In order to oppose the Jews, he (salAllahu alayhi wa sallam) resolved to observe one more Saum (9th of Muharram) with that of 10th of Muharram and ordained it for the Muslims.
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