Musannaf Ibn Abi Shayba 8:825

Abu Bakr narrated to us, he said: 'Abdah bin Sulayman narrated to us, from Yahya bin Sa'id, from Nafi', from Ibn 'Umar, he said: The Prophet, peace and blessings of Allah be upon him, would come and say: "Labbayk Allahumma Labbayk, Labbayk la sharika laka Labbayk, inna al-hamda wa al-ni'mata laka wa al-mulk, la sharika lak." (At your service, O Allah, at your service. At your service, there is no partner for You, at your service. Indeed, all praise and grace are Yours, and the dominion; there is no partner for You.)