I requested a reference from a prof whom I used to know . He politely declined my request and responded this way:
Hmmm..it is tricky - since I have not taught you or worked with you - it would be very hard for me to say anything about your work in a fair and honest manner. I believe that you have done very good work - but you must understand that only someone who has worked with you or taught you can give a strong recommendation - one that will convince the reviewers.
If I give you a recommendation - it will have very little value since I would have only an indirect knowledge of your work and would not be able to write convincingly. I hope you can understand.
I hope you understand why I am reluctant to write a letter for you.
I wish you all the best.
I feel this is totally fair and what he said is right. I want to thank him for the long email he wrote to decline my request in a nice and kind manner. How should I go about it?