Musnad Ahmad 27:548

Abu Mu'awiyah narrated to us, Ubaydullah narrated to us, from Nafi', from Ibn Umar, who said: Four [phrases] I received from the Messenger of Allah, may Allah bless him and grant him peace: "Labbayk Allahumma Labbayk, Labbayka la sharika laka Labbayk, inna al-hamda wa an-ni'mata laka, wa al-mulka la sharika laka." (Here I am, O Allah, here I am. Here I am, there is no partner for You, here I am. Indeed, all praise and grace are Yours, and the dominion; there is no partner for You.)