Musnad Ahmad 337:12

Sufyan narrated to us, from Abdullah ibn Abi Bakr, from Abd al-Malik ibn Abi Bakr ibn al-Harith, from Khallad ibn al-Sa’ib ibn Khallad, from his father, from the Prophet, peace and blessings of Allah be upon him, who said: "Gabriel, peace be upon him, came to me and said: 'Command your companions to raise their voices with the *ihlal* (the Talbiyah).'"