Abd al-Rahman bin Yazid narrates: When Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) intended to prostrate, he would level the pebbles with his hand once only, and in his prostration he would say: "Labbayka wa sa'dayka (Here I am, and ready to obey You)."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 9259