Fiqh-us-Sunnah Volume 5, Hajj and ‘Umrah, Fiqh 5.074.

Thus if someone were to walk within the Hijr (Commonly known as Hijr Isma’il, and situated to the north of Ka’bah, it is surrounded by a semicircular wall. Of this, only an area of six cubits (three metres approximately) extending from the Ka’bah is its part) his tawaf will not be valid, because both Hijr and Al-Shazarwan (Al-Shazarwan is the construction contiguous to the Ka’bah to which are fixed the rings holding the Ka’bah cover) are a part of the Ka’bah. Allah has commanded us to circumambulate around the Ka’bah and not inside it, saying: “…and circumambulate the Ancient House.” (Qur’an 22.29) It is preferable, however, to remain close to the Ka’bah, if one can do so easily.