مُسْنَدُ أَحْمَدَ
Musnad Ahmad 2045
Musnad of 'Abdullah bin al-'Abbas bin 'Abdul-Muttalib from the Prophet
'Uthman bin al-Hakam said: I asked Sa'eed bin Jubair about fasting in Rajab: what do you think about it? He said: Ibn 'Abbas told me: The Messenger of Allah (ﷺ) would fast until we thought that he would not break his fast, and he would refrain from fasting until we thought that he would not fast.
Musnad Ahmad 2046Its isnad is saheeh]