Sahih Muslim Book 007, Hadith Number 3075.
Chapter : The merit of entering the ka'ba for a pilgrim and observing of prayer in it.
Ibn Juraij reported: I said to 'Ata': Have you heard Ibn 'Abbas saying: You have been commanded to observe circumambulation, and not commanded to enter it (the Ka'ba)? He ('Ata') said: He (Ibn Abbas) (at the same time) did not forbid entrance into it. I, however, heard him saying: Usama b. Zaid informed me that when Allah's Apostle (may peace be upon him) entered the House, he supplicated in all sides of it; and he did not observe prayer therein till he came out, and as he came out he observed two rak'ahs in front of the House, and said: This is your Qibla. I said to him: What is meant by its sides? Does that mean its corners? He said: (In all sides and nooks of the House) there is Qibla.