Majma al-Zawaid 2783 — What to say in bowing and prostration

Abd al-Rahman bin Yazid said: Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) would level the pebbles once with his hand when he was about to prostrate, and in his prostration he would say: "Labbayka wa sa'dayka (Here I am at Your service, and happiness can only be attained from You)."
Imam al-Tabarani recorded it in al-Mu'jam al-Kabir; its narrators are the narrators of the Sahih.