Hazrat Moosa (alaihis salaam) asking Allah Ta'ala why He gave Imaan to few people

Q: I have heard many times a narration where Moosa (alaihis salaam) asked Allah Ta'ala why he gave imaan to a few people and not all. Allah Ta'ala told him to go into a garden and choose a flower. He chose the best flower so Allah Ta'ala said that He chose Imaan for who He loved the most.

Could you tell me if this is authentic?

Making mistakes during Ibaadat

Q: I know a person who has many problems and difficulties when it comes to performing salaah, reciting Quraan and making zikr. He has a bad memory, poor concentration and experiences difficulty in pronouncing Arabic words properly. He makes many mistakes when performing salaah and reciting the Quraan. Is it still necessary for him to carry out ibaadaat such as salaah and reciting the Quraan despite making so many mistakes?