Sahih Bukhari Volume 2, Book 26, Hadith Number 719.

Narated By ‘Abdullah bin Masud : I offered (only a) two Rakat prayer with the Prophet (at Mina), and similarly with Abu Bakr and with ‘Umar, and then you d offered in opinions. Wish that I would be lucky enough to have two of the four Rakat accepted (by Allah).