Referring to a result that comes later So in writing a math paper, if I am writing a proof of a theorem and I need to refer a lemma that comes later in the same paper. What is the proper way 
to say that? For example if I am proving a theorem in page 10 and I need a lemma 
in page 15, how should I say that in the proof?
The proper way here is to rearrange your paper so that this does not happen. It might be possible to put the theorem into the paper twice. Once in the introduction, possibly without some technical details but with more context and intuition around it and once after the required lemmata a proven with all the technical details. 

The main reason is that the proofs of your various lemmata and theorems depend on one another but you want the dependence to form a tree and not have any loops. If you have a theorem that depends on a lemma proved later on, it could look like the proof of the lemma also depends on the theorem stated earlier on.