Answered by Sidi Mostafa Azzam
I have read the position of the shafii school (refering to the Reliance) that the follower must recite Al-Fatiha when following the imam and can lag up to 3 integrals, can you explain this in more detail inshallah? Also is the follower allowed in the situations of wasawasa or forgeting to recite to omit fatiha and have the salah still be intact?
As The Immaculate Raiment says, “You must recite the whole Fatihah in the standing of every cycle (rak`ah) of every prayer, whether praying alone or in a group, unless in that cycle was insufficient time to recite the whole Fatihah behind the imam, because of his quick recital or because of your coming late, in which case you recite as much as you can (making up for any lost time), then bow.”
The upshot is that it is essential (fard) for you to recite the Fatihah, unless you are following an imam behind whom there was not enough time to recite it (i.e., at a medium pace). In such a case, you are not obligated to recite the rest of the Fatihah; rather, you are to bow with the imam.
You should not lag behind the imam without excuse. If the imam recites quickly and you lacked enough time when standing behind him to recite the entire Fatihah at a medium pace, then you do not complete it; rather, you bow without completing it. You are not excused for lagging behind him in such a scenario.
The reference in Reliance of the Traveller to lagging behind the imam three long physical integrals only applies to someone with an excuse to lag behind. Excuses to lag behind include having forgetten to recite the Fatihah, being naturally a slow reciter, though not excessive waswasah.
If the follower lags behind the imam by two physical integrals without excuse, his prayer is invalidated.
Also see the following well-answered answer: http://groups.yahoo.com/group/shafii/message/245 by my brother Sidi Moustafa Elqabbany.
And Allah knows best.