It was the practice of the Prophet (peace and blessings be upon him):
"When he (peace and blessings be upon him) bowed, he would keep his back so level that if water were poured on it, it would stay in place."
Hadith Referenceصحیح الجامع الصغیر / حرف الكاف / 4732
Hadith Gradingالألبانی:صحيح
Hadith Takhrij[هـ] عن وابصة [طب] عن ابن عباس وأبي برزة وأبي مسعود. الروض النضير ٧٨، صفة الصلاة ١١١: عم، طس.