Muhammad himself said he was being possessed until Khadija gave her fatwa after this very interesting spirit test:
Ismail b. Abu Hakim, a freedman of the family of al-Zubayr, told me on Khadija's authority that she said to the apostle of God, 'O son of my uncle, are you able to tell me about your visitant, when he comes to you?" He replied that he could, and she asked him to tell her when he came. So when Gabriel came to him, as he was wont, the apostle said to Khadija, 'This is Gabriel who has just come to me.' Get up, O son of my uncle,’ she said, and sit by my left thigh.' The apostle did so, and she said, Can you see him?’ Yes,' he said. She said, Then turn around and sit on my right thigh.’ He did so, and she said, Can you see him?' When he said that he could she asked him to move and sit in her lap. When he had done this she again asked if he could see him, and when he said yes, she disclosed her form and cast aside her veil while the apostle was sitting in her lap. Then she said, Can you see him?’ And he replied, No.' She said, O son of my uncle, rejoice and be of good heart, by God he is an angel and not a satan.’
a spirit not ashamed of the lower parts of Khadija but of her face so Khadija’s testimony is that it must be an angel and not satan as Muhammad thought.
Now going back to our question : do Muslims associate partners with Allah when they say the Quran is pre-existent and uncreated ?