One key quotation from John Wesley that comes up in various contexts is:
You have nothing to do but to save souls. Therefore spend and be spent in this work.
This is a valid quote, but the original source is rarely cited. It is found in the document Minutes of Several Conversations Between The Rev. John Wesley, M.A. and Others from the Year 1744 to the Year 1789. In the document, look to pages 16 – 17. These are the Rules of a Helper, and the entire list is worth reading. You are looking for Question 26, and the quote in question here is listed as #11. The full quote is:
You have nothing to do but to save souls. Therefore spend and be spent in this work. And go always, not only to those that want you, but to those who want you most.