Language:
كتاب الفتن
92  
Afflictions and the End of the World
(20)
Chapter: “This son of mine is a chief, and Allah may make peace between two groups of Muslims through him.”
(20)
بَابُ قَوْلِ النَّبِيِّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ لِلْحَسَنِ بْنِ عَلِيٍّ: «إِنَّ ابْنِي هَذَا لَسَيِّدٌ وَلَعَلَّ اللَّهَ أَنْ يُصْلِحَ بِهِ بَيْنَ فِئَتَيْنِ مِنَ الْمُسْلِمِينَ»
Sahih al-Bukhari 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).

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.‏
Reference : Sahih al-Bukhari 7110
In-book reference : Book 92, Hadith 57
USC-MSA web (English) reference : Vol. 9, Book 88, Hadith 226
  (deprecated numbering scheme)