Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) said: That barrier (the Hijr) is part of the House of Allah, because the Messenger of Allah (peace and blessings be upon him) performed tawaf of the House of Allah from behind it, and Allah Almighty has said: "And let them perform tawaf of the Ancient House." [Surah Al-Hajj: 29]
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحج / 9319