Safiyyah bint Shaybah narrates: "A woman from Banu Abdud-Dar, the daughter of Abi Tijarah, told me that I came with the women of Quraysh to the house of Abi Al-Husayn, and we were watching the Messenger of Allah ﷺ performing Sa'i between Safa and Marwah. I saw him performing Sa'i, and his lower garment was twisting due to the intensity of the Sa'i, to the extent that I say I saw his knees. And I heard him saying: 'Perform Sa'i! Indeed, Allah has made Sa'i obligatory upon you.'"
Hadith Referenceمسند الإمام الشافعي / كتاب الحج / 981