Taking off the reference number from in text cite

I have a question about how to take off the reference number from in text cite? to explain more, I am using a thesis template, which uses the bibtex. This is how I cite a reference in text:

\citep[Rost and Riebesell, 2004]{rost2004coccolithophores}

the previous line produce a number before the Author names as follow: ....(1, Rost and Riebesell, 2004) and when I use additional square brackets the number appears to the right of the names:

\citep[Rost and Riebesell, 2004][]{rost2004coccolithophores}

the cite will appear as follow:

....(Rost and Riebesell, 2004 1)

I want to take off this number from the in text cite to be like this:

....(Rost and Riebesell, 2004)

So I hope someone will guide me to the right way to do this?