Idol Worship
Polytheism is infact a belief system. Polytheistic actions like praying to a deity other than Allah are a result of that belief system. But in Islam, just the way polytheism as a belief system is prohibited similarly, adopting symbols of polytheism like bowing/prostrating/praying to an idol are also prohibited. Quran 22:30 condems them both in a way where both the belief and the action seem to be equated: فَاجۡتَنِبُوا الرِّجۡسَ مِنَ الۡاَوۡثَانِ وَ اجۡتَنِبُوۡا قَوۡلَ الزُّوۡرِ “… So guard yourselves against the filth of these idols and abstain from falsehoods which you fabricate about them regarding God “. ( Excerpt from Quran 22:30 ) It is a very serious act for which one might be held accountable on the day of judgement depending on the excuse/reason for such an act which may or may not be acceptable in the sight of God. The comparison with a Buddhist situation is not accurate since such an act is not done in front of a known idol which is worshipped, this situation involves a bow of respect. In Islam, even bow and prostration of respect which look like ruku and Sajda have also been prohibited on grounds of Sadd-e-Zariya but it is still different than bowing to an idol.