Condition for following the Imam
There are 14 conditions when following the Imam.
1. The Niyyat of the follower (Muqtadi) is to be simultaneous with the Tahreema.
2. Females following will be valid if the Imam makes Niyyat of leading them.
3. The Imam’s heel has to be ahead of the Muqtadi’s.
4. Condition of Imam not to be inferior to that of the Muqtadi’s.
5. For the Imam not to be performing a different Fardh than the Muqtadi.
6. For the Imam to be a resident whilst the Muqtadi is performing his Qaza Salaat i.e. two rakaats.
7. There is no row of woman between the Imam and the followers.
8. There is no river between the Imam and the followers. (a river in which small boats sail).
9. There is no road upon which carts (cars, etc.).
10.Nor a wall, which causes confusion, regarding the Imaam’s movement. If there is no confusion due to him being audible or visible, then it is correct to follow him.
11.The Imam should not be mounted whilst the followers are on foot (or vise versa).
12.The follower should also not be mounted on a different animal/vehicle other than the Imam’s.
13.The follower should not be in one boat and the Imam be in another boat, which is not attached to the follower’s.
If attached then following is valid.
14.If according to the follower’s opinion, the Imam’s Salaat has broken by flowing blood or vomiting a mouth full and he is sure that the Imam did not repeat his ablution then it is not valid to follow the Imam.
It is valid for one who has made Wudhu to follow someone who has made Tayammum, also for one who has washed the limb to follow one who has made Masah, (over bandage, etc) also for one standing to follow one who is sitting and for a hunch back or one performing Salaat by signs to follow another of the same type; and for one performing Nafl to follow one performing Fardh.
If it became known that the Imam’s Salaat was nullified, the followers should try his best to notify all the followers to repeat their Salaat.
Source: Taleem ud Deen by Shaykh Mufti Afzial Hoosen Elias.