Is it legally obligatory to inquire about the source of unknown funds offered for investment in a permissible (halal) venture for a known percentage of the profit, given that there is a suspicion about their origin, such as being smuggled or laundered money?

It appears that the money in a person's hand is their property, but strong circumstantial evidence indicates that the money was smuggled and its owner is unknown. It is unlikely that large sums would be transferred without prior knowledge, which suggests that the money is illicit (haram). Therefore, the questioner must verify the permissibility of these funds. If it is highly probable that they are illicit, then one must refrain from them, because in this case, strong probability takes the place of certainty. Simply asking the person mentioned about the source of his money is not sufficient, as his statement cannot be relied upon.