Dear…
__(I/We)__ regret to inform you that __(I/our company)__ cannot accommodate your request for a letter of recommendation at this time. While it is true that you maintained a satisfactory level of performance during your time with __(Name of Organization)__, __(I/we)__ don’t believe that __(I/our company)__can fairly evaluate your abilities based on only a __(length of time)__ period of employment.
You do appear to be a capable individual and __(I/we)__ wish you all the best in your future endeavors.
Yours truly,