Sahih Bukhari Volume 5, Book 57, Hadith Number 26.
Narated By Ibn ‘Abbas : While I was standing amongst the people who were invoking Allah for Umar bin Al-Khattab who was lying (dead) on his bed, a man behind me rested his elbows on my shoulder and said, “(O ‘Umar!) May Allah bestow His Mercy on you. I always hoped that Allah will keep you with your two companions, for I often heard Allah’s Apostle saying, “I, Abu Bakr and ‘Umar were (somewhere). I, Abu Bakr and ‘Umar did (something). I, Abu Bakr and ‘Umar set out.’ So I hoped that Allah will keep you with both of them.” I turned back to see that the speaker was Ali bin Abi Talib.