This paper discusses the possible security risks caused by not using Hash in RSA digital signature scheme.
If the hash function is not used, the receiver cannot know whether the message has been modified. If you use the hash function, you can see whether the message has been modified, because you use the hash function to calculate the message digest first. If the message has been modified, the receiver can't get the corresponding digest when calculating the hash message digest of the received message. Hash functions are generally one-way hashes, so it is generally impossible to calculate messages from hash digests.