بسم الله الرحمن الرحيم
(Fatwa: 539/504/SN=08/1436)
If there is a sutra (barrier) in front of the imam i.e. he is leading salah facing a poll or tree or has arranged for sutra by planting some stick etc then it is allowable to pass beyond the sutra though there is no barrier in front of those offering salah behind him. In case there is no sutra in front of the imam then it is makrooh and a sin to pass in front of him. A Hadith states that if the one who passes in front of a person who is praying knew what (burden of sin) there is on him, standing for forty years would be better for him than passing through.
لو يعلم المار بين يدي المصلي ماذا عليه كان لأن يقوم خريفاً خير له من أن يمر بين يديه (البحرالزخار بمسند البزار ، رقم 3782).
However, if salah is offered in a big house or a mosque (whose length is sixty feet from mehrab to yard) or salah is being offered in ground then it is allowable to pass in front of two or three rows and it shall cause no sin. It is recorded in Halabi Kabiri:
ويكره المرور بين يدي المصلي إذا لم يكن عنده حائل نحو السترة أو الأسطونة أو نحوها (ص: 366، اشرفى)
It is recorded in Dur Mukhtar 2/203:
وكفت سترة الإمام للكل ، وانظر : رد المحتار على الدر المختار (2/398، زكريا) وإعلاء السنن (5/80-81 ، حكم المرور بين يدي المصلي الخ)
Allah knows Best!
Darul Ifta,
Darul Uloom Deoband