Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) narrates that the Messenger of Allah (peace and blessings be upon him) entered the Ka'bah, and Sayyiduna Bilal bin Rabah, Sayyiduna Usamah bin Zayd and Sayyiduna Uthman bin Talhah al-Hajabi (may Allah be pleased with them) also entered with him. They closed the door, and he (peace and blessings be upon him) remained inside for some time. Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) said: When they came out, I asked Sayyiduna Bilal (may Allah be pleased with him): What did the Messenger of Allah (peace and blessings be upon him) do inside the Ka'bah? He said that he (peace and blessings be upon him) placed one pillar to his left, two pillars to his right and three pillars behind him, and prayed. In those days the Ka'bah stood on six pillars.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 3781