Rust Command Problems

let file_path = "my.test".to_string();

let mut file = File::create(file_path).unwrap();

file.write_all("ls".as_bytes());

let mut process = Command::new("source")
                  .arg(file_path)
                  .spawn()
                  .unwrap_or_else(|e| { panic!("failed to execute child: {}", e) });

Returns an error:

thread '<main>' panicked at 'failed to execute child: No such file or directory (os error 2)'

but if I change the command from source to ls then it finds the file