Sayyiduna Jabir bin Abdullah (may Allah be pleased with them both) said: When the Messenger of Allah (peace and blessings be upon him) came down from Safa, he would walk, until when his feet reached the middle of the valley, he would run (sa'y), until he came out of it.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحج / 9336