Shaddad bin Maqil and I went in to see Ibn Abbas, and Ibn Abbas said, "The Messenger of Allah ﷺ left behind nothing except what lies between these two covers."Then we went in to see Muhammad bin Ali, and he said the same thing.He added that Mukhtar used to claim that revelation came to him.
حدثنا سفيان حدثنا
دخلت أنا وشداد بن معقل على ابن عباس فقال ابن عباس ما ترك رسول الله ﷺ إلا ما بين هذين اللوحين ودخلنا على محمد بن علي فقال مثل ذلك قال وكان المختار يقول الوحي
Its isnad is sahih. Abdul-Aziz bin Rafi, with damma on the raa, is a trustworthy tabii. Shaddad bin Maqil is a tabii. Muhammad bin Ali is Ibn Hanafiya, as is made explicit in Bukhari's narration. Bukhari related the hadith in 9:58 from Qutayba from Sufyan.