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. (Using translation from Abū Dāʾūd 2430)   

عَنْ صَوْمِ رَسُولِ اللهِ ﷺ تَطَوُّعًا قَالَ كَانَ يَصُومُ حَتَّى نَقُولَ لَا يُفْطِرُ وَيُفْطِرُ حَتَّى نَقُولَ لَا يَصُومُ  

[Machine] About fasting in the month of Rajab, how do you see it? He said, Ibn Abbas narrated to me that the Messenger of Allah ﷺ used to fast until we would say, "He does not break his fast," and he would break his fast until we would say, "He does not fast."  

عَنْ صَوْمِ رَجَبٍ كَيْفَ تَرَى فِيهِ ؟ قَالَ حَدَّثَنِي ابْنُ عَبَّاسٍ أَنَّ رَسُولَ اللهِ ﷺ كَانَ يَصُومُ حَتَّى نَقُولَ لَا يُفْطِرُ وَيُفْطِرُ حَتَّى نَقُولَ لَا يَصُومُ