Sayyiduna Wabisah bin Ma'bad (may Allah be pleased with him) said: I saw the Messenger of Allah (peace and blessings be upon him) that when he (peace and blessings be upon him) bowed (ruku') in his prayer, if water had been poured on his (peace and blessings be upon him) back, it would have stayed in place (that is, his (peace and blessings be upon him) back was so straight and level).
Hadith Referenceالمعجم الكبير للطبراني / باب الواو / 18620