Sahih Bukhari Volume 9, Book 88, Hadith Number 226.

Narrated By Harmala : (Usama’s Maula) Usama (bin Zaid) sent me to ‘Ali (at Kufa) and said, “‘Ali will ask you, ‘What has prevented your companion from joining me?’ You then should say to him, ‘If you (‘Ali) were in the mouth of a lion, I would like to be with you, but in this matter I won’t take any part.’ ” Harmala added: “‘Ali didn’t give me anything (when I conveyed the message to him) so I went to Hasan, Hussain and Ibn Ja’far and they loaded my camels with much (wealth).”