Dear Felix
We are considering [NAME, ex. James Johnson] for the position of [POSITION, ex. Sales Manager] at [FIRM NAME, ex. Power Account Limited]. As [he/she] has identified you as a reference, I would be delighted if you could provide us with your professional opinion as to [his/her] strengths and weaknesses. {STATE OTHER REQUESTED INFORMATION, ex. We are interested in his managerial skills, his abilities to work in teams, and any other information you deem relevant.}
I thank you in advance and look forward to your reply.
Regards
Lydia