Musannaf Ibn Abi Shayba 34:176

'Affan narrated to us, Hammad bin Salamah narrated to us, from 'Ali bin Zayd, from 'Ubayd Allah bin Ibrahim, who said: The first person to cast pebbles in the Mosque of the Prophet (may Allah bless him and grant him peace) was 'Umar bin al-Khattab. When the people would raise their heads from prostration, they would shake their hands. So he ordered pebbles, and they were brought from al-'Aqiq, and they were spread in the Mosque of the Prophet (may Allah bless him and grant him peace).