Is it permissible to use cowrie shells for adornment or accessories, given the Hadith: "Whoever hangs a cowrie shell, may Allah not grant him peace" regarding it, and should this Hadith be acted upon even if it is weak?

Wada' is a white object extracted from the sea. It is forbidden to hang it to ward off the evil eye, as that is considered polytheism (shirk). So, whoever believes that it inherently wards off harm without Allah's command, this is major shirk. And if one believes it is a means to ward off harm, but it is not a means sanctioned by religious law (Sharia) or by common custom, then this is minor shirk.

However, using wada' and shells for decorating homes without any corrupt belief is permissible, because the basic principle regarding things is permissibility unless there is evidence of their prohibition. This is based on the generality of Allah's saying: (It is He who created for you all of that which is on the earth) Al-Baqarah/29, and (Say, "Who has forbidden the adornment of Allah which He has produced for His servants and the good things, of provision?" Say, "They are for those who have believed in [this] worldly life [and] exclusively for them on the Day of Resurrection.") Al-A'raf/32.

However, if it becomes widespread among people that wada' is used to ward off the evil eye, then a Muslim should not wear it or adorn their home with it, even if they do not intend to ward off the evil eye. This is to avoid resembling those who intend the forbidden, and to block the means to ill-suspicion and following error.