Sayyidah Habibah (may Allah be pleased with her) said: I looked out through an opening between Safa and Marwah and saw the Messenger of Allah (peace and blessings be upon him). He (peace and blessings be upon him) was performing the sa'i between Safa and Marwah and saying to his Companions: "Perform the sa'i, for Allah has made the sa'i obligatory upon you." If only you had seen how, from the intensity of the sa'i, the waist-wrapper was swirling around his (peace and blessings be upon him) stomach and thighs, so much so that I saw the whiteness of his (peace and blessings be upon him) thighs.
Hadith Referenceالمعجم الكبير للطبراني / مسند النساء / 20946