All perfect praise be to Allaah, the Lord of the worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and messenger.
In a narration on the authority of Anas the companions, despite their great love for the Prophet, sallallaahu ‘alayhi wa sallam, never stood up when seeing him coming, for they knew how much he, sallallaahu ‘alayhi wa sallam, hated that. [Al-Bukhari and At-Tirmithi]
Indeed, standing up is an act of glorification, and in principle it must be reserved solely for Allaah Alone, The Lord of the Worlds. It is for this reason that it is the best action during prayer. This is supported by a narration on the authority of Jaabir where the Prophet, sallallaahu ‘alayhi wa sallam, mentioned that the best action in the prayer is standing up for a long time. [Ahmad and Muslim]
Thus, if the Prophet, sallallaahu ‘alayhi wa sallam, who is the best and most honorable person in the whole world, hated seeing people standing up for his sake, how can a person stand up for a detestable religious innovation taken from non-Muslim traditions. In a narration on the authority of Abu Sa‘eed Al-Khudri the Prophet, sallallaahu ‘alayhi wa sallam, mentioned that the Muslims will slip into the same sins that the Jews and Christians have slipped into and will copy their acts, even if such acts contradict the Sharee‘ah. [Al-Bukhari and Muslim]
Therefore, it is impermissible to stand up while the national anthem is sung. Moreover, it is the duty of Muslims to seek what is right in their customs and traditions and to make sure they are all governed by Sharee‘ah-approved criteria, and not blindly imitating the non-Muslim nations. In a narration on the authority of Ibn ‘Umar the Prophet, sallallaahu ‘alayhi wa sallam, mentioned that when a person imitates a people, he becomes one of them. [Abu Daawood and At-Tabaraani]
Allaah Knows best.