Musannaf Ibn Abi Shayba 34:130

Abu Usama narrated to us, from Hisham, from al-Hasan, from his father, from a man from Thaqif, who said: A man from Thaqif consulted Umar about pebble-paving the mosque. He said, "O Commander of the Faithful, it is softer for the feet and more concealing for phlegm and mucus." Umar said, "Pave it with pebbles from the blessed valley, from al-Aqiq." So, the first one to pebble-pave the mosque was Umar, may Allah be pleased with him.