In ruku, one should recite سُبْحَانَ رَبِّيَ الْعَظِيْم thrice. One may also recite the following dua in ruku when performing sunnat or nafl salaah:
سُبْحَانَكَ اللَّهُمَّ رَبَّنَا وَبِحَمْدِكَ اللَّهُمَّ اغْفِرْ لِي
Glory be to You, O Allah, our Rabb, with all Your praise! O Allah, forgive me.
عن عائشة رضي الله عنها، قالت: كان النبي صلى الله عليه وسلم يقول في ركوعه وسجوده: سبحانك اللهم ربنا وبحمدك اللهم اغفر لي (صحيح البخاري، الرقم: 794)
Hazrat Aaishah (radhiyallahu anha) reports that Rasulullah (sallallahu alaihi wasallam) would recite the following in his ruku and sajdah (in his nafl salaah):
سُبْحَانَكَ اللَّهُمَّ رَبَّنَا وَبِحَمْدِكَ اللَّهُمَّ اغْفِرْ لِي