Will Allah give a greater paradise to one who has faith equal to a grain of mustard seed?

Mufti Sahab, Assalam Alekum First of all sorry for asking too many Question within a week. Mufti Sahab I want to ask you that in tablighi Jamaat everybody is telling that ‘Jo Raee ke dane ke baraabar bhi Imaan leke jayega usko Allah paak Duniya se 10 guni badi jannat dega.’ Mufti sahab every body in jamaat who is old or new use this hadith, but i think 99% people dont know the quantity of ‘Raee ke dane ke baraabar Imaan kitna hota hai’. Even I dont know. I want to know that either Imaam Gajhali or Mullah Ali Qari has explained this hadith i.e. ‘How will that person who will have Raee ke dane ke baraabar imaan’. In Fatwa Id 9421 as I told One of my friend Dr. Warsi (done PHD in Fiqa from Al-Azahar Egypt). He is Mureed of Imaam Ghajali R.A. He told me that Imaam Ghajali has explained this in his book. I asked this to one Mufit has also but he told that he has only 1 book of Imaam Ghajali