What is the ruling
    on kissing another person's hand?
Question

                What is the ruling on kissing another
      person's hand?
Praise be to Allah.It is mustahabb to kiss the hands of righteous people
    and the greatest scholars, and it is makrooh to kiss the hand of anyone else. The hand of
    the beardless man should not be kissed under any circumstances, 
(From Fatawa al-Imaam al-Nawawi, p. 71)
In the commentary on the book, it says:
If a person wants to kiss another person's hand, if this is because of
    his asceticism, righteousness, knowledge, position or modesty, or other reason that has to
    do with his commitment to religion, then it is not makrooh. In such cases it is mustahabb,
    because Abu Ubaydah kissed the hand of Umar (may Allaah be pleased with them
    both).
But if it is because of the persons riches, wealth, high rank or
    status in the eyes of the people of this world, and so on, then it is intensely makrooh.
Al-Mutawalli, one of our companions, said: It is not permitted, and he
    indicated that it is haraam.
It was narrated in Sunan Abi Dawood that Zaari (may Allaah
    be pleased with him), who was among the delegation of Abd Qays, said: We
    started to get out from our howdahs and we kissed the hands and feet of the Prophet 
    (peace and blessings of Allaah be upon him).