Musnad Ahmad 1267:2

Surayj narrated to us, he said: Abdullah ibn al-Mu'ammal narrated to us, from 'Ata' ibn Abi Rabah, from Safiyyah bint Shaybah, from Habibah bint Abi Tajrah, who said: I saw the Messenger of Allah (peace and blessings of Allah be upon him) performing Tawaf between al-Safa and al-Marwah, while the people were in front of him and he was behind them. He was hastening (sa'y) until I could see his knees from the intensity of the hastening, and his lower garment was swirling around him. He was saying: "Hasten, for Allah has prescribed the hastening for you."