Sahih Bukhari Volume 002, Book 021, Hadith Number 268.
Narated By Mujahid : Somebody came to the house of Ibn 'Umar and told him that Allah's Apostles had entered the Ka'ba. Ibn 'Umar said, "I went in front of the Ka'ba and found that Allah's Apostle had come out of the Ka'ba and I saw Bilal standing by the side of the gate of the Ka'ba. I said, 'O Bilal! Has Allah's Apostle (p.b.u.h) prayed inside the Ka'ba?' Bilal replied in the affirmative. I said, 'Where (did he pray)?' He replied, '(He prayed) Between these two pillars and then he came out and offered a two Rakat prayer in front of the Ka'ba.' " Abu 'Abdullah said: Abu Huraira said, "The Prophet (p.b.u.h) advised me to offer two Rakat of Duha prayer (prayer to be offered after sunrise and before midday). " Itban (bin Malik) said, "Allah's Apostle (p.b.u.h) and Abu Bakr, came to me after sunrise and we aligned behind the Prophet (p.b.u.h) and offered two Rakat."