ما هو حجر إسماعيل؟

The Hijr Ismail is the northern part of the Kaaba that the Quraysh fell short of including in its construction, so they placed a curved wall around it to indicate that it is part of the House. This is evidenced by Aisha's (may Allah be pleased with her) question to the Prophet (peace be upon him) about the wall, to which he replied, "Yes," and informed her that her people's funds fell short. Its naming as "Hijr Ismail" is a common, not a legal, appellation. This part, enclosed by the wall, is indeed part of the House, and circumambulation is not valid without it.