Musnad Ahmad 30:1377

Abu Sa'id narrated to us, 'Abd al-'Aziz ibn 'Abd Allah narrated to us, 'Abd Allah ibn al-Fadl narrated to us, from al-A'raj, from Abu Hurayrah, who said: It was among the talbiyah of the Prophet, may Allah bless him and grant him peace: "Labbayka ilaha al-haqq" (At Your service, O God of Truth).