ElGamal encryption security Assume we have 2 prime numbers in form p = 2q + 1.
Is it safe to use the cyclic group of order p-1 instead of one with order q in ElGamal encryption? I couldn't find any suitable source to explain why we should use q as order of our group and avoid p-1.
Most websites and sources just say:

use cyclic group of order q

without any explanation.
What have I tried?
I assumed p = 11 and therefore q = 5. I did all calculations of ElGamal-Encryption by considering 10 as my group order($Z_{p}^{*}$):

Group(G) = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10}
Group Order(q): 10
*Generator(g): 7
Message(m): 9

and there was no problem in decrypting the message. So I think there is something wrong with its security!
I thinks there is no need to mention the calculations using q = 10 But if you need them, just comment below the question to bring them into the question as an update.
The answer to your question can be found in the paper Why Textbook ElGamal and RSA Encryption Are Insecure by Boneh et al. It is an easy paper to read and worthwhile going through the details there.