Musannaf Ibn Abi Shayba 3:6457

Abu Bakr narrated to us, he said: Ibn Numayr narrated to us, he said: Hisham ibn Urwah narrated to us, from his father, that Umar intended not to pebble the mosque. Sufyan ibn Abdullah al-Thaqafi signaled to him, saying: "Yes, O Commander of the Faithful, for it is more concealing for the phlegm and softer for the sitting place." So Umar said: "Pebble it."