Urwah (may Allah have mercy on him) said: The first to spread pebbles in the mosque, the mosque of the Messenger of Allah (peace and blessings be upon him), was Sayyiduna Umar bin al-Khattab (may Allah be pleased with him). He said: "Spread it with pebbles from the blessed valley," meaning al-Aqiq.
This is how Urwah narrated it; the hadith of Ibn Umar is muttasil (connected), and there is no harm in its chain.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4314