Whenever Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) performed the circuits of Safa and Marwah, he would begin with Safa and climb it until the House of Allah came into view, and he would say three takbirs and say: "La ilaha illallahu wahdahu la sharika lah, lahu al-mulku wa lahu al-hamd, wa huwa 'ala kulli shay'in qadir" (There is no god but Allah, alone, without partner; to Him belongs the dominion and to Him belongs all praise, and He has full power over all things). He would do this seven times, making twenty-one takbirs and seven tahlils. Then he would supplicate in between and ask of Allah. Then he would come down until, when he reached the middle of the valley, he would run, until he came up out of it; then he would walk on to Marwah and climb it, and do just as he had done on Safa. He would do this seven times until he had finished his sa'y.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحج / 9344