When you use a source in your writing, demonstrate your evaluation of the source's reliability by carefully choosing a signal verb to show your understanding of the author's purpose, intentions and the degree to which he affirms his statement. By using signal verbs, you let readers know the context in which the source's statement should be viewed. For example, if you choose the signal verb say instead of prove, you are reporting that the author makes his statement without any substantial support.
Choose your signal verbs carefully so that they genuinely reflect the tone and substance of each cited source.