I know that it is Haram to stand for the national anthem. Please give me clear evidence from the Qur'an and Hadith so that I can prove that to someone else.

Praise be to Allah, the Lord of the World; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

As you have mentioned, it is Haram to stand for the national anthem. A prohibition of such an action depends on:
1) This action constitutes a form of glorification and sanctification, which is like worship.
2) This action does not only establish the principles of allegiance and loyalty on nationalism and patriotism, but also it is at the expense of allegiance to Allah, Islam and Muslims.
3) The national anthem is accompanied with music.
4) Such an action constitutes an imitation of Jews and Christians.
Muslims have to be distinguished among others nations and have to avoid the others' special characteristics.
Allah knows best.