Right click your project and select ‘Open Module Settings’ under SDK Location put your location for your SDK.
paste in /Users/AhmadMusa/Library/Android/sdk
Clean and rebuild your project
Update
Try to delete your local.properties file and create a new one, but do not check it into version control.
Right click top level of project and Create new file ‘local.properties’
then add: sdk.dir=/Users/AhmadMusa/Library/Android/sdk
Clean and build