Is it permissible to associate another with Allah Almighty in a vow, such as by saying, "The slaughter is for Allah, and its reward is for Husayn"? And is it permissible to perform an act for the reward of the righteous and the Family of the Prophet?

It is not permissible to associate anyone other than Allah the Almighty in any act of worship, such as vows, because the Prophet, peace and blessings be upon him, said in the Hadith Qudsi: "I am the One Who is most free from the need of partners. Whoever performs an action in which he associates another with Me, I will abandon him and his shirk (associating partners with Allah)."

Vowing to Husayn or any other Companion or member of the Household of the Prophet is not permissible, because it is an act of worship, and directing it to other than Allah is shirk (polytheism).

However, sacrificing for Allah the Almighty, giving the meat in charity, and dedicating the reward of the charity to the righteous, or the Household of the Prophet, or other Muslims, is permissible.