While in the bowing position, at least once, and preferrably three or more times, you should say:
Transliteration: subḥaana rabbiyal-‛adheem
Translation: How far from imperfections my Lord is, The Supreme
It was narrated that Ḥudhayfah (may Allah be pleased with him) said:
"I prayed with the Messenger of Allah (may the peace and blessings of Allah be upon him), and he bowed and said when bowing: 'subḥaana rabbiyal-‛adheem' (How far from imperfections my Lord is, The Supreme)..." [an-Nasā'ī: 1046]
There are also other supplications which can be made after the first one, while in the bowing position, such as:
Transliteration: subḥaanak-allaahumma rabbanaa wa biḥamdik, allaahumm-aghfir lee.
Translation: How far from imperfections You are O Allah, our Lord and I praise You. O Allah, forgive me.
and
Transliteration: subbooḥun quddoos, rabbul-malaa’ikati war-rooḥ.
Translation: Perfect and Holy (He is), Lord of the angels and the Rooḥ (i.e. Jibreel).
and
Transliteration: allaahumma laka raka‛tu wa bika aamantu, wa laka aslamt,
khasha‛a laka sam‛ee, wa baṣaree, wa mukh-khee, wa ‛aẓmee, wa ‛aṣabee, wa mastaqallat bihi qadamee.
Translation: O Allah, unto You I have bowed, and in You I have believed, and to You I have submitted.
My hearing, and my sight, and my mind, and my bones, and my tendons, and what my feet carry, are humbled before You.
There are several other authentic narrations which are collected in the books of hadeeth.
Note that it is not permissible to recite the Qur'an in rukoo' or in sujood because the Prophet (may the peace and blessings of Allah be upon him) said:
"I have indeed been forbidden from reciting the Qur'an in rukoo' or sujood. Therefore, in the rukoo', glorify the supremity of your Lord, Mighty and Sublime, and as for the sujood, exert yourselves in supplication, for it is most likely that you will be answered." [an-Nasā'ī: 1046]