When Muslims go into rukoo while offering their daily Salah, they recite Subhaana Rabbiyal-'Adheem (Glory to my Lord the Exalted) three times. This dua is one of the best ways to proclaim the glory of Allah (سُبْحَانَهُ وَتَعَالَى) while praying. It is an integral part of salah; the daily salah is incomplete without reciting this dua in rukoo. It is a great way to declare the glory and perfection of Allah (سُبْحَانَهُ وَتَعَالَى) while standing in front of Him.
The Prophet Muhammad (ﷺ) while praying. The dua declares the glory of Allah (سُبْحَانَهُ وَتَعَالَى) and praises Him (Sunan an-Nasa'i 1145). Hudhaifah(رَضِيَ ٱللَّٰهُ عَنْهُ) reported about this dua in a separate event in (Sunan an-Nasa'i 1133) and (Sunan an-Nasa'i 1046). The dua while rukoo has also been narrated by Aishah (رَضِيَ ٱللَّٰهُ عَنْهَا) in (Sunan an-Nasa'i 1122) and (Sunan an-Nasa'i 1123).
Prayer Duas
Subhaana Rabbiyal-'Adheem.
Glory to my Lord the Exalted. (Three times)
Subhaanaka Allaahumma Rabbanaa wa bihamdika Allaahum-maghfir lee.
Glory is to You, O Allah, our Lord, and praise is Yours. O Allah, forgive me.
Subboohun, Quddoosun, Rabbul-malaa'ikati warrooh.
Glory (to You), Most Holy (are You), Lord of the angels and the Spirit.
Allaahumma laka raka'tu, wa bika 'aamantu, wa laka 'aslamtu khasha'a laka sam'ee, wa basaree, wa mukhkhee, wa 'adhmee, wa 'asabee, wa mastaqalla bihi qadamee.
O Allah, to You I bow (in prayer) and in You I believe and to You I have submitted. Before You my hearing is humbled, as is my sight, my mind, my bones, my nerves and what my feet have mounted upon (for travel).
Prayer is when the believer stands before his creator, knowing the Almighty is listening to him. At such a moment, making a dua that voices the glory of Allah (سُبْحَانَهُ وَتَعَالَى) is an extremely potent way of attaining forgiveness of one's sins from Him in addition to multitudes of blessings and sustenance of Allah. Thus, while making this dua, one must fully concentrate and should wholeheartedly recite the dua.
Reciting the dua while in rukoo is a ticket to forgiveness from the Almighty. If you recite this dua, Allah Azzawajal will increase your good deeds and shower blessings upon you. Most notably, He will accept your salah as salah is incomplete without reciting this dua in rukoo. The dua reminds us of our place in the world, the sovereignty of Allah (سُبْحَانَهُ وَتَعَالَى), and our ultimate dependence on Him for every small thing in life.