Musannaf Ibn Abi Shayba 27:188

Waki’ narrated to us, from Sufyan, from Salamah, from Mujahid, regarding the verse: "And proclaim to the people the Hajj." He said: Ibrahim was commanded to proclaim the Hajj, so he stood and said: "O people, answer your Lord." So they answered him: "Labbayk Allahumma Labbayk."