Does a slight backward arching of the back when rising from rukūʿ invalidate the prayer?

Rising from bowing (ruku') is a pillar (rukn), and the prayer is not valid without it. What suffices for it is for the person to stand upright. A slight leaning forward or backward does not harm, as long as it does not deviate from the state of standing. Doubting the leaning does not invalidate the prayer, for the default is the existence of uprightness.