In ruku, one should recite سُبْحَانَ رَبِّيَ الْعَظِيْم thrice.
Reciting the following tasbeeh in ruku is also established in the sunnah:
سُبْحَانَ ذِي الْجَبَرُوتِ وَالْمَلَكُوتِ وَالْكِبْرِيَاءِ وَالْعَظَمَةِ
Glory be to (Allah Ta’ala), the Possessor of all might, the Possessor of the entire kingdom, the Possessor of all grandeur, the Possessor of all greatness.
عن عوف بن مالك قال: قمت مع رسول الله صلى الله عليه وسلم ليلة، فلما ركع مكث قدر سورة البقرة يقول في ركوعه: سبحان ذي الجبروت والملكوت والكبرياء والعظمة (سنن النسائي، الرقم: 1049)
Hazrat Awf bin Maalik (radhiyallahu anhu) says: “I stood with Rasulullah (sallallahu alaihi wasallam) on one night to perform (nafl) salaah. When he went into ruku, he remained in ruku for the duration of reciting Surah Baqarah, while reciting the following tasbeeh (in ruku):
سُبْحَانَ ذِي الْجَبَرُوتِ وَالْمَلَكُوتِ وَالْكِبْرِيَاءِ وَالْعَظَمَةِ