I wanted to verify the requirements in Hanafi fiqh to catch a rak'ah in jama'ah?

In the name of Allah, the most beneficent, the most merciful.

Answer

An individual will be regarded as performing the rakah if he joins the imam in ruku before the imam lifts his head from ruku.

In Raddul Muhtar, Vol1, P533, it states that a person will not be regarded as a masbuk (latecomer) if he manages to do ruku before the imam lifts his head from ruku.

(2) To what extent is one allowed to lag behind the imam in Jama’ah (all four madhahib)?

Hanafi Fiqh

The extent to which one is allowed to lag behind the Imam will depend on the act itself. If the muqtadi (follower) is lagging behind due to his completion of a rukun/wajib then he should complete this rukun/wajib before joining the Imam, As an example, if the muqtadi is reciting tashahud and the Imam has already stood up, then in this situation the muqtadi should finish his tashahud before joining the Imam.

However if the act is a Sunnah then he should join the Imam immediately. For example, if the muqtadi is reciting the tasbeeh of ruku and the Imam has already stood up then the muqtadi should join him straight away. (Raddul Muhtar, Vol1, P347)

 Shafee Fiqh

If the muqtadi deliberately lags behind the Imam by two rukuns then the salah will become void otherwise on the fourth rukun he should join the imam and make up what he missed after the salaam of the imam. (Kitabul Fiqh Alal Mazhabil Arbaah, VOL1, P279 & 385)

Maliki Fiqh

If the muqtadi lags behind the imam by one rukun then the salah will become void if the following two conditions are met.

(i)                  The incident took place in the first rakah, and

(ii)                the act was done deliberately.

(Kitabul Fiqh Alal Mazhabil Arbaah, VOL1, P382)

Hambali Fiqh

If the muqtadi deliberately lags behind the imam by one rukun then the salah will become void.  If the lagging occurred due to forgetfulness or a valid excuse then the salah will not become void. In this situation the muqtadi should complete the rukun as long as he does not anticipate missing the next rakah. If there is a possibility of that happening then he should join the imam and complete this rakah after the salaam of the imam. (Kitabul Fiqh Alal Mazhabil Arbaah, VOL1, P384)

(3) If one joined the imam in the first rakah in jama’ah, can one remain sitting (completing one’s regular invocations) even after the imam’s salam.(all madhahibs with sources please for all questions)

Hanafi Fiqh

If the muqtadi has recited tasahhud then he should do salaam with the imam. If the muqtadi has not completed the tasahhud then he should complete this before doing salaam. (Raddul Muhtar, Vol1, P347)

 Shafiee & Hambali Fiqh

As above. (Kitabul Fiqh Alal Mazhabil Arbaah, VOL1, P214)

Malki Fiqh

Do salaam with the imam as the tasahhud is sunnah. (Kitabul Fiqh Alal Mazhabil Arbaah, VOL1, P214)

And Allah knows best.

Sayeedur Rahman

Darul Ifta, Birmingham