I am applying for PhD programmes very soon and I have recently been trying to get in touch with my potential referees. At my previous institution there is a professor who knows me the most out of all academics I know; we had plenty of conversations regarding the field I want to pursue my PhD in and also I did very well in modules taught by him. I was planning to request a reference from him and I tried to get in touch with him three times (through e-mail), however, he has not replied to me (it has been about two months since my first e-mail). Should I assume that he overlooked my e-mails (he is very busy) and send him another message? Or rather, should I try to contact him through the Department? It might seem to be a trivial case, but I really do not want to annoy him too much as his reference would potentially be of the biggest importance in my applications. What would you suggest in this particular case?