Musnad Ahmad 30:1195

Rawh narrated to us, Usamah ibn Zayd narrated to us, he said: Abdullah ibn Abi Labid narrated to me, from al-Muttalib ibn Abdullah ibn Hantab, he said: I heard Abu Hurayrah saying: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Gabriel commanded me to raise the voice in the *ihlal* (the Talbiyah), for it is from the symbols of Hajj."