The World Federation One Stop Fiqh

Ruling 2238

The task that the offeror wishes to be performed for him must not be unlawful (ḥarām), pointless, or an obligatory (wājib) task that must legally be performed free of charge. Therefore, if a person offers £100 to whoever drinks wine, wanders into a dark place at night without any rational purpose, or performs his obligatory prayers (ṣalāh), the reward is not valid.