Musnad Ahmad 30:1508

Hujayn ibn al-Muthanna Abu Umar narrated to us, Abd al-Aziz—meaning Ibn Abd Allah ibn Abi Salamah al-Majishun—narrated to us, from Abd Allah ibn al-Fadl, from Abd al-Rahman al-A’raj, from Abu Hurayrah, who said: It was among the Talbiyah of the Messenger of Allah, peace and blessings of Allah be upon him: "Labbayka ilaha al-haqq" (At Your service, O God of Truth).