If a person joins the Salah while the Imam is in Ruku` before raising his head, (Part No. 6; Page No. 405) it will be as if he had attended a complete Rak`ah. The evidence for this is the Hadith reported by
Abu Bakrah Al-Thaqafy (may Allah be pleased with him)
"He reached the Prophet (peace be upon him) in the mosque while he was bowing in prayer, then he bowed before joining the row and mentioned it to the Prophet (peace be upon him). The Prophet said to him: May Allah increase your love for the good, but do not repeat it again (bowing in that way)."
This Hadith was reported by
The Hadith clearly indicate the valid realization of the Rak`ah, as the Prophet (peace be upon him) did not ask him to repeat it. The basic rule in legislation is generality. This is said by the Four Imams (Imams Abu Hanifah, Malik, Al-Shafi`y, and Ahmad) and the majority of scholars.May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.