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.  

أَنَّ رَسُولَ اللَّهِ ﷺ كَانَ يَصُومُ حَتَّى نَقُولَ لاَ يُفْطِرُ وَيُفْطِرُ حَتَّى نَقُولَ لاَ يَصُومُ