Police are investigating the stabbing of a man at a Rebels bikie gang clubhouse in Sydney's west overnight. Police say the man in his mid 20s was stabbed in the upper body during a fight outside the clubhouse at Jamisontown, near Penrith. Officers are investigating the cause of the fight. Police believe the injured man is an associate of the Rebels gang. Inspector Gary Sims says the injured man is now in hospital. "He is at Westmead Hospital in a stable condition," Inspector Sims said. "Police are dismayed at the level of violence in this attack and we are using all resources to try and investigate the cause." Inspector Sims says police are not yet sure how many people were involved in the fight. "We believe at this stage it's people associated with the Rebels outlaw motorcycle gang," he said. "Investigators are following a number of lines of inquiry at this stage. "It's too early to say how we're progressing but they are following every opportunity."