From Sahih Bukhari (Hadith 7110):
Narrated 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)
Reference: Bukhari 7110