What is the authenticity of the following narration?
When it is raining if you recite this salutation [durud] once, for every drop of rain that falls, you will receive ten good deeds, ten of your deeds will be forgiven and your rank will increase by ten:
اللهم صل علي محمد بعدد قطرات الامطار
Allahumma salli ‘ala Muhammadin bi ‘adadi qataratil amtar
O Allah! Send salutations upon Muhammad as many as the number of raindrops