مُسْنَدُ أَحْمَدَ
Musnad Ahmad 2986
Musnad of 'Abdullah bin al-'Abbas bin 'Abdul-Muttalib from the Prophet
‘Uthman bin Hakeem said: I asked Sa'eed bin Jubair about fasting Rajab. What do you think about it? He said: Ibn ‘Abbas (رضي الله عنه) told me that the Messenger of Allah (ﷺ) used to fast until we said he will not stop fasting, and he used not to fast until we said he will not fast.
Musnad Ahmad 3009Its isnad is saheeh]