An important thing here is that you bring evidence of a stipend being promised for your position. If you have the job advertisement saved that mentions a stipend, or any other document that specifically mentions a stipend for you position, point this out in your email. For example, "In the job advertisement, it was mentioned that a stipend would be provided for lab assistants. I just wanted to clarify that this is true and ask when and how this stipend will be given out."