Hadithcore
Sahih al-Bukhari, 7110
sahih

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).

حَدَّثَنَا عَلِيُّ بْنُ عَبْدِ اللَّهِ، حَدَّثَنَا سُفْيَانُ، قَالَ قَالَ عَمْرٌو أَخْبَرَنِي مُحَمَّدُ بْنُ عَلِيٍّ، أَنَّ حَرْمَلَةَ، مَوْلَى أُسَامَةَ أَخْبَرَهُ قَالَ عَمْرٌو وَقَدْ رَأَيْتُ حَرْمَلَةَ قَالَ أَرْسَلَنِي أُسَامَةُ إِلَى عَلِيٍّ وَقَالَ إِنَّهُ سَيَسْأَلُكَ الآنَ فَيَقُولُ مَا خَلَّفَ صَاحِبَكَ فَقُلْ لَهُ يَقُولُ لَكَ لَوْ كُنْتَ فِي شِدْقِ الأَسَدِ لأَحْبَبْتُ أَنْ أَكُونَ مَعَكَ فِيهِ، وَلَكِنَّ هَذَا أَمْرٌ لَمْ أَرَهُ، فَلَمْ يُعْطِنِي شَيْئًا، فَذَهَبْتُ إِلَى حَسَنٍ وَحُسَيْنٍ وَابْنِ جَعْفَرٍ فَأَوْقَرُوا لِي رَاحِلَتِي‏.‏

References2 variants
In-Book Reference
Book 92, Hadith 57
USC-MSA web (English) reference
Vol. 9, Book 88, Hadith 226 (deprecated numbering scheme)