Musnad Ahmad 1175:1

'Abbad ibn al-'Awwam narrated to us, from Hilal—meaning ibn Khabbab—from 'Ikrimah, from Ibn 'Abbas, that Duba'ah bint az-Zubayr ibn 'Abd al-Muttalib came to the Prophet, may Allah bless him and grant him peace, and said: "O Messenger of Allah, I intend to perform Hajj, so should I make a condition?" He said: "Yes." She said: "Then how should I say it?" He said: "Say: 'Labbayk Allahumma Labbayk (Here I am, O Allah, here I am), my place of exiting Ihram is wherever You restrain me.'"