Muhaqqaq Masala

If a person catches the Imam in Ruku (bowing) even for a moment before the Imam lifts his head from Ruku, even if this moment is less than one Tasbeeh, he will be considered to have caught that Rak'ah. However, if the Imam is in the state of rising from Ruku, and the Muqtadi (follower) is in the state of going into Ruku, then he will not be considered to have caught the Rak'ah; therefore, it will be obligatory for him to repeat the Rak'ah. (Aham Masail / Vol: 3 / Pg: 72)