Jim's draft will be perfectly sufficient.
However, for your future reference, Fatemah, I'll mention that in 'business English', it's always considered best to use a person's name (including any academic title, where relevant) where it's known, rather than to use 'Dear Sir' or 'Dear Madam'. So, for example, "Dear Professor Schmidt" or "Dear Dr Müller" is considered much better than "Dear Sir' or "Dear Madam".
Enquiries after a person's health (such as "I hope you are well") or how they're doing (e.g. "I hope that you're having a good week") are considered by some to be inappropriate unless you've already established a personal relationship with the recipient of your correspondence.
My own preference for the content of your email would to be ask (again) for the letter itself (which is what really matters), rather than for an indication of when it might arrive.
So my own wording would be along these lines:
"Dear Professor Schmidt,
I write to remind you that I am awaiting a letter from you, confirming that I am currently enrolled for study at the University of Siegen and fully participating in my studies. I need this letter to reapply for my visa to study in Germany, after my initial application was rejected.
I would be most grateful if you would provide me with this letter at your earliest convenience, as I am becoming concerned that without it I might be forced to abandon my studies.
I offer my thanks in anticipation of your kind assistance in this matter.
Regards
Fatemah X"