Is it permissible to sprinkle water behind a traveler when they depart?

Bidding farewell to a traveler by sprinkling water on them is a custom that has no basis in Islam and is considered a reprehensible innovation (bid'ah). It may even escalate to polytheism (shirk) if the one who does it believes that it is a cause for protecting the traveler. The Sunnah is to bid them farewell by supplicating for them, asking for piety, forgiveness of sins, and facilitation of good, as reported from the Prophet, peace and blessings be upon him.

Making something a cause when it is not a legal (shar'i) or predetermined (qadari) cause can lead to shirk. The causes known to humans are of two types: those established by religious law and those established by empirical experience. Whoever establishes a cause that Allah has not made a legal or predetermined cause has made himself a partner with Allah.

Wearing a ring, thread, or similar items to ward off or remove affliction, if the wearer believes they inherently affect things without Allah, is major shirk. If they believe it is merely a cause, then it is minor shirk.