Narrated Al-Bara: The bowing, the prostration the sitting in between the two prostrations and the standing after the bowing of the Prophet but not qiyam (standing in the prayer) and qu`ud (sitting in the prayer) used to be approximately equal (in duration).
Abu Humayd (may Allah be pleased with him) said to his companions that the Prophet (peace and blessings of Allah be upon him) performed rukū‘, then bent his back completely.
Hadith Referenceصحيح البخاري / كتاب الأذان (صفة الصلوة) / 792