Musnad Ahmad 32:21

Hashim narrated to us, Humaid al-Tawil informed us, Bakr bin Abdullah al-Muzani informed us, he said: I heard Anas bin Malik narrating, he said: I heard the Prophet, peace and blessings of Allah be upon him, pronouncing the Talbiyah for Hajj and Umrah together. I narrated that to Ibn Umar, and he said: He pronounced the Talbiyah for Hajj alone. So I met Anas and told him what Ibn Umar said, and he said: You do not consider us as anything but children! I heard the Messenger of Allah, peace and blessings of Allah be upon him, saying: "Labbayka Umratan wa Hajjan" (At Your service, an Umrah and a Hajj).