Mujahid said that Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) asked Sayyiduna Bilal (may Allah be pleased with him) about the prayer of the Prophet (peace and blessings be upon him) inside the Ka'bah, and he told him: "He (peace and blessings be upon him) prayed two rak'ahs in the middle of the House, kept the pillar to his right, moved forward a little, and kept the Maqam (of Ibrahim) behind his back."
Hadith Referenceالمعجم الكبير للطبراني / باب الباء / 1026