Hadith 14:118
Sahih (Al-Albani)
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
Original Reference: Sunan Abi Dawud 2430