Is ascending fully onto Safa and Marwah obligatory during Sa'i (the ritual walk)?

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It is obligatory to fully cover the area between Safa and Marwah. The pilgrim must place his heel at the base of Safa and proceed to Marwah. If one cannot ascend Marwah, placing the toes at the base suffices. However, ascending fully upon both hills is superior, following the example of the Prophet, peace be upon him. Failing to complete any part of the distance between them, even a short measure, invalidates the Sa'i unless that distance is covered.