Many Chinese learners have trouble with using the word 了 correctly. I can tell you that you should leave it out in this sentence, but I don't know exactly why we should leave it out. It just sounds natural to my ears without the word 了。 You can use the word 了 in a sentence such as :"我终于收到了他的来信。" , which in English would be :"I finally receive his letter." You anticipated this incoming letter from him, so you can use 了. I'm not sure whether we use it because of an anticipation or from a known source (in this case him). I'm sure there are other situations in which you would use 了, but I'm no Chinese grammarian, so I don't know.