Language:
كتاب المناقب
61  
Virtues and Merits of the Prophet (pbuh) and his Companions
(23)
Chapter: The description of the Prophet (saws)
(23)
باب صِفَةِ النَّبِيِّ صلى الله عليه وسلم
Sahih al-Bukhari 3542

Narrated `Uqba bin Al-Harith:

(Once) Abu Bakr offered the `Asr prayer and then went out walking and saw Al-Hasan playing with the boys. He lifted him on to his shoulders and said, " Let my parents be sacrificed for your sake! (You) resemble the Prophet (ﷺ) and not `Ali," while `Ali was smiling.

حَدَّثَنَا أَبُو عَاصِمٍ، عَنْ عُمَرَ بْنِ سَعِيدِ بْنِ أَبِي حُسَيْنٍ، عَنِ ابْنِ أَبِي مُلَيْكَةَ، عَنْ عُقْبَةَ بْنِ الْحَارِثِ، قَالَ صَلَّى أَبُو بَكْرٍ ـ رضى الله عنه ـ الْعَصْرَ، ثُمَّ خَرَجَ يَمْشِي فَرَأَى الْحَسَنَ يَلْعَبُ مَعَ الصِّبْيَانِ، فَحَمَلَهُ عَلَى عَاتِقِهِ وَقَالَ بِأَبِي شَبِيهٌ بِالنَّبِيِّ لاَ شَبِيهٌ بِعَلِيٍّ‏.‏ وَعَلِيٌّ يَضْحَكُ‏.‏
Reference : Sahih al-Bukhari 3542
In-book reference : Book 61, Hadith 51
USC-MSA web (English) reference : Vol. 4, Book 56, Hadith 742
  (deprecated numbering scheme)