Sayyidah Bint Abi Tajra'ah (may Allah be pleased with her), one of the women of Banu Abd al-Dar, said: I entered the house of the family of Abu Husayn with some women of Quraysh, and the Prophet (peace and blessings be upon him) looked towards me while he (peace and blessings be upon him) was performing the sa'i between Safa and Marwah. I saw him (peace and blessings be upon him) performing the sa'i in the bottom of the valley, and his (peace and blessings be upon him) waist-wrapper was swirling from the intensity of the sa'i, so much so that I would say that I could see his (peace and blessings be upon him) knee. And I heard him (peace and blessings be upon him) say: "Perform the sa'i, for Allah has made the sa'i obligatory upon you."
Hadith Referenceالمعجم الكبير للطبراني / مسند النساء / 20943