Sahih Bukhari Volume 8, Book 73, Hadith Number 173.

Narated By Abu Salama bin ‘Abdur-Rahman bin ‘Auf : That he heard Hassan bin Thabit Al-Ansari asking the witness of Abu Huraira, saying, “O Abu- Huraira! I beseech you by Allah (to tell me). Did you hear Allah’s Apostle saying’ ‘O Hassan ! Reply on behalf of Allah’s Apostle. O Allah ! Support him (Hassan) with the Holy Spirit (Gabriel).’?” Abu Huraira said, “Yes.”