Musannaf Ibn Abi Shayba 21:519

Narrated to us Abdullah ibn Numayr, from Ubaydullah, from Nafi, from Ibn Umar, that when he would ascend al-Safa, he would face the House (the Kaaba), then say "Allahu Akbar" (God is the Greatest) three times. Then he would say: "There is no god but Allah alone, He has no partner; to Him belongs the dominion, and to Him belongs the praise, and He is capable of all things," raising his voice with it. Then he would supplicate for a little while, then he would do that at al-Marwa, until he had done that seven times. Thus, the takbir (saying "Allahu Akbar") would be twenty-one takbirs. He would hardly finish until it became difficult for us, even though we were young.