Shama-il Tirmidhi Chapter 055, Hadith Number 004 (392).

Kulayb bin Shihaab Al-Kufi says that Abu Hurayrah Radiyallahu 'Anhu narrated to me the mubaarak saying of Rasulullah Sallallahu 'Alayhi Wasallam, that whomsoever sees me in a dream, has really seen me, because the shaytaan cannot impersonate me. Kulayb says, I mentioned this hadith to Ibn 'Abbaas Radiyallahu 'Anhu and also told him that I was blessed with the seeing of Rasulullah Sallallahu 'Alayhi Wasallam in my dream. At that moment I thought of Hasan bin 'Ali Radiyallahu 'Anhu. I said to Ibn 'Abbaas Radiyallahu 'Anhu that I found the image in my dream very similar to that of Hasan Radiyallahu 'Anhu. Ibn 'Abbaas Radiyallahu 'Anhu verified my statement, that truly Hasan Radiyallahu 'Anhu was very similar to Rasulullah Sallallahu 'Alayhi Wasallam in appearance.

Commentary
In some narrations it has been related that the chest and the portion above it, of the body of Sayyidina Hasan Radiyallahu'Anhu was similar to that of Sayyidina Rasulullah Sallallahu 'Alayhi Wasallam, and the lower portion of Sayyidina Husayn Radiyallahu 'Anhu was similar in appearance to that of Sayyidina Rasulullah Sallallahu'Alayhi Wasallam.