For a worshipper inside Makkah who can directly see the Ka'bah, what is the mandatory requirement for Istiqbal al-Qiblah?

Conditions of Prayer Part 2: Facing the Qiblah (استقبال القبلة)

• Facing the general direction of Makkah.
• Facing the exact physical structure of the Ka'bah itself ('Ayn al-Ka'bah).
• Facing the east direction.
• Facing the mountain of Safa.