Dua to be recited at the Sacred Site i.e. Al-Mash'ar Al-Haram is not specified during hajj, however, you (Pilgrim) can say any dua explaining, greatness of Allah Subhana Wa Ta’ala, His oneness and can be supplicated to Allah as well.

As it is narrated from Hazrat Jabir (RA) that he said,
 

“The messenger of Allah (peace and blessings of Allah be upon him) rode Al-Qaswa (His Camel) until he reached Al-Mash'ar Al-Haram, he then faced the qiblah (the Kaba), supplicated to Allah, and extolled His greatness and oneness. He stood until the sun shone but left before it rose.”