Narrated 'Uthman b. Hakim:I asked Sa'id b. Jubair about fasting during Rajab. He said: Ibn 'Abbas told me that the Messenger of Allah (ﷺ) used to fast to such an extent that we thought that he would never break his fast; and he would go without fasting to such an extent that we thought he would never fast.
حَدَّثَنَا إِبْرَاهِيمُ بْنُ مُوسَى، حَدَّثَنَا عِيسَى، حَدَّثَنَا عُثْمَانُ، - يَعْنِي ابْنَ حَكِيمٍ - قَالَ سَأَلْتُ سَعِيدَ بْنَ جُبَيْرٍ عَنْ صِيَامِ رَجَبَ، فَقَالَ أَخْبَرَنِي ابْنُ عَبَّاسٍ، أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم كَانَ يَصُومُ حَتَّى نَقُولَ لاَ يُفْطِرُ وَيُفْطِرُ حَتَّى نَقُولَ لاَ يَصُومُ .
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Fasting (Kitab Al-Siyam) • Hadith 2430