Musnad Ahmad 337:9

'Affan narrated to us, he said: Hammad ibn Salamah narrated to us, he said: Muhammad ibn Ishaq informed us, from 'Abdullah ibn Abi Labid, from al-Muttalib ibn 'Abdullah ibn Hantab, from al-Sa'ib ibn Khallad: "That Gabriel, peace be upon him, came to the Prophet, may Allah bless him and grant him peace, and said: 'Be one who raises his voice (al-'ajj) and one who sacrifices (al-thajj).'" And al-'ajj is the talbiyah (saying "Labbayk"), and al-thajj is the slaughtering of the sacrificial animals (al-budn).