Musnad Ahmad 25:500

Ya'qub narrated to us, my father narrated to us, from Ibn Ishaq, Khasif ibn 'Abd al-Rahman al-Jazari narrated to us, from Sa'id ibn Jubayr, who said: I said to 'Abdullah ibn 'Abbas: "O Abu al-'Abbas, I am amazed at the difference of opinion among the companions of the Messenger of Allah (may Allah bless him and grant him peace) regarding the *ihlal* (entering into the state of ihram) of the Messenger of Allah (may Allah bless him and grant him peace) when he made it obligatory." He said: "I am the most knowledgeable of people regarding that. It was only one Hajj from the Messenger of Allah (may Allah bless him and grant him peace), and that is where they differed: The Messenger of Allah (may Allah bless him and grant him peace) went out for Hajj. When he prayed his two rak'ahs in his mosque at Dhu al-Hulayfah, he made it obligatory while in his sitting place, and he pronounced the *tahlil* (saying 'Labbayk') for Hajj when he finished his two rak'ahs. Some people heard that from him and memorized it from him. Then he mounted, and when his she-camel stood up with him, he pronounced the *tahlil*, and some people caught that from him. This is because the people were coming in groups. So they heard him when his she-camel stood up with him pronouncing the *tahlil*, and they said: 'The Messenger of Allah (may Allah bless him and grant him peace) only pronounced the *tahlil* when his she-camel stood up with him.' Then the Messenger of Allah (may Allah bless him and grant him peace) proceeded, and when he ascended the height of al-Bayda', he pronounced the *tahlil*, and some people caught that from him. They said: 'The Messenger of Allah (may Allah bless him and grant him peace) only pronounced the *tahlil* when he ascended the height of al-Bayda'.' By Allah, he indeed made it obligatory in his place of prayer, and he pronounced the *tahlil* when his she-camel stood up with him, and he pronounced the *tahlil* when he ascended the height of al-Bayda'. So whoever takes the statement of 'Abdullah ibn 'Abbas, let him pronounce the *tahlil* in his place of prayer when he finishes his two rak'ahs."