What is the ruling on one who says "Subhana Rabbiyal A'la" (Glory be to my Lord, the Most High) during bowing (rukūʿ), and is it permissible to pray behind him?

The authentic Sunnah agrees that a worshiper says in their bowing (ruku'): "Subhana Rabbiyal-Azeem (Glory be to my Lord, the Most Great)," and in their prostration (sujud): "Subhana Rabbiyal-A'la (Glory be to my Lord, the Most High)." This is confirmed by the Prophet's actions (peace be upon him) and his commands. Muslim narrated from Hudhaifah (may Allah be pleased with him) that he prayed with the Prophet (peace be upon him), and the Prophet would say in his bowing: "Subhana Rabbiyal-Azeem," and in his prostration: "Subhana Rabbiyal-A'la." Likewise, Ahmad, Abu Dawud, and Ibn Majah narrated from Uqbah ibn Amir (may Allah be pleased with him) that the Prophet (peace be upon him) said upon the revelation of Allah's saying: (So glorify the Name of your Lord, the Most Great) [56:74]: "Make it in your bowing." And upon the revelation of Allah's saying: (Glorify the Name of your Lord, the Most High) [87:1]: "Make it in your prostration."

The jurists (fuqaha') differed regarding the ruling on glorifying Allah (tasbeeh) in bowing and prostration. The majority consider it to be recommendable (mustahabb), while Ahmad, Ishaq, and Dawud held it to be obligatory (wajib), which is the more preponderant view. Those who hold this view substantiate it with the Prophet's command (peace be upon him) to do it, and his saying: "Pray as you have seen me praying." According to the view of obligation, if someone intentionally omits the tasbeeh, or deviates from it by saying in bowing: "Subhana Rabbiyal-A'la," their prayer is invalid due to their intentional omission of an obligation. However, according to the view of the majority, their prayer is valid. It is appropriate to advise those who deviate from the Sunnah and to be keen on adhering to it.