If you don't straighten your back in ruku, in Salah, is your salah invalid?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.

The sufficient and valid Rukoo‘ is for the worshipper to bow so that his hands can touch his knees, but the most complete and perfect Rukoo‘ is for the worshipper to grip his knees with his fingers spread, making the back level and the head parallel with it, and keeping his arms away from his sides. If the worshipper performed Rukoo‘ in the most perfect manner, he gains the full reward of the recommended performance of Rukoo‘.

The Hanbali book Ar-Rawdh Al-Murbi‘ ma‘a Zaad Al-Mustaqni‘ reads,

“The worshipper should place his hands on his knees, and it is recommended to spread out his fingers … The back should be straight (in a horizontal position), the head parallel with it (on the same level) … Waabisah ibn Ma‘bad said, "I saw the Prophet, sallallaahu ‘alayhi wa sallam, perform prayer, and when he bowed he made his back so straight that if water were poured on it, it would have stayed there." [Ibn Maajah] The arms should be away from the sides; and the sufficient Rukoo‘ is to bend forward so that his hands touch his knees.” For more benefit, please refer to Fatwa 133195.

Hence, the worshipper’s failure to make his back straight (in a horizontal position) in Rukoo‘ does not invalidate the prayer, as long as he performs the sufficient and valid Rukoo‘ described above.

Allaah Knows best.