Sunan Abi Dawud 2430
سنن أبي داود#2430SahihNarrated '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
Scholarly Authentication
We show the most cautious grade (weakest-wins) as the overall label. Individual scholars graded it as follows:
Al-AlbaniSahih
Muhammad Muhyi Al-Din Abdul HamidSahih
Shuaib Al ArnautSahih
Zubair Ali ZaiSahih Bukhari (1971 Mukhtasar) Sahih Muslim (1157)
In-book reference: 14:118