Mujahid (may Allah have mercy on him) says that the Prophet (peace and blessings be upon him) would recite the talbiyah aloud: 'Labbayk Allahumma labbayk' (Here I am, O Allah, here I am). He mentioned the complete talbiyah, then said: Until one day, when the people were going back, it was as though the sound of the talbiyah pleased him, and he (peace and blessings be upon him) added these words to it: 'Labbayk inna al-'aysha 'ayshu al-akhirah' (Here I am; indeed, the true life is the life of the Hereafter). Ibn Jurayj (may Allah have mercy on him) says: I think that was the Day of Arafah.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحج / 9035