Praise be to Allah.
With regard to kissing the hand, the majority of scholars are of the view that it is makrooh, especially if it is a habit. But if it is done sometimes on some occasions, there is nothing wrong with that in the case of a righteous man, a righteous ruler, a father and the like. There is nothing wrong with that, but if it is done regularly then it is makrooh.
Some of the scholars regarded that as haraam if it is customarily done at every meeting. But if it is done occasionally then there is nothing wrong with it.
As for putting the forehead to the hand as a kind of prostration on the hand, this is a haraam kind of prostration and the scholars call it the lesser prostration. It is not permissible to put the forehead to the hand of the person to signify prostrating on it. That is not permissible. But kissing it with the mouth, if that is not done as a habit, but rather is done rarely or occasionally, there is nothing wrong with it because it is narrated from the Prophet (blessings and peace of Allah be upon him) that some of the Companions kissed his hands and feet. The matter is not serious if it is done rarely, but if it becomes habitual and is always done then it is makrooh or haraam.
As for bowing, that is not permissible. If it is like rukoo’ (bowing in prayer) it is not permissible, because bowing is an act of worship and it is not permissible to bow. But if the bowing is not an act of veneration, rather he bowed because he is short and the one who is greeting him is tall, so he leans forward to shake hands with him, and not because of veneration; rather it is so that he can greet him if he is short or is paralyzed -- then there is nothing wrong with that. But if he bow or leans forward out of respect for him, that is not permissible, and there is the fear that it may be shirk if the intention is to show respect and honour thereby.
It was narrated that the Prophet (blessings and peace of Allah be upon him) was asked: O Messenger of Allah, when I meet a man should I bow to him? He said: No. He said: Should I embrace him and kiss him? He said: No. He said: Should I shake his hand? He said: Yes. Even though the isnaad of this hadeeth is weak, it should be followed, because there is a great deal of corroborating evidence that supports its meaning. Similarly, there is a great deal of evidence that bowing to people is not permissible.
To sum up, it is not permissible at all to bow to any person, kings or otherwise, but if it is not done as a sign of veneration, rather a person leans forward to greet another if he is short or paralyzed or is sitting down, so he leans forward to greet him, there is nothing wrong with that. End quote.
Shaykh ‘Abd al-‘Azeez ibn Baaz (may Allah have mercy on him)