From Sahih Bukhari (Hadith 1657):
Narrated `Abdullah bin Mas`ud:I offered (only a) two rak`at prayer with the Prophet (at Mina), and similarly with Abu Bakr and with `Umar, and then you differed in opinions. Wish that I would be lucky enough to have two of the four rak`at accepted (by Allah)
Reference: Bukhari 1657