In ruku, one should recite سُبْحَانَ رَبِّيَ الْعَظِيْم thrice.
Reciting the following tasbeeh in ruku is also established in the sunnah:
سُبُّوحٌ قُدُّوسٌ رَبُّ الْمَلَائِكَةِ وَالرُّوحِ
You are glorified, You are pure, You are the Rabb of the angels and the Rabb of Jibreel (alayhis salaam)
عن عائشة قالت: كان رسول الله صلى الله عليه وسلم يقول في ركوعه: سبوح قدوس رب الملائكة والروح (سنن النسائي، الرقم: 1048)
Hazrat Aaishah (radhiyallahu anha) reports that Rasulullah (sallallahu alaihi wasallam) would recite the following tasbeeh in his ruku
سُبُّوحٌ قُدُّوسٌ رَبُّ الْمَلَائِكَةِ وَالرُّوحِ