Musannaf Ibn Abi Shaybah 31618 — The supplication on climbing Safa

Nafi' said that when Sayyiduna Ibn Umar (may Allah be pleased with them both) climbed the hill of Safa, he would face the House of Allah, then say the takbir three times, then recite these words: "There is no god but Allah, He is alone, He has no partner; His is the dominion and to Him belongs all praise, and He has power over all things." He would raise his voice with these words. Then he would supplicate for a little while, then do the same on the hill of Marwah, until he had made seven such rounds, so the number of takbirs came to twenty-one. Though we were young men, we would be very tired by the time he was close to finishing.