Is refusing gifts or food from relatives and friends, due to not being entirely certain about the source of their money (especially if there is a suspicion, such as usurious bank interest), a blameworthy misgiving or a legitimate act of piety? And what is the extent of the authenticity of the widely circulated story about the father of one of the Four Imams, which is used to demonstrate the danger of illicit wealth and its impact on children?

Seeking out what is lawful is commendable, but it should not reach the point of extremism and excessive scrupulosity. One should not refrain from accepting a gift from someone if it is not known with certainty that their wealth contains anything unlawful. This is an act of extremism and innovation in religion, for a Muslim whose affairs are concealed (i.e., not known to be involved in anything unlawful) should not be treated with suspicion in their dealings.

One must be wary of harboring ill suspicions about people, for the default is that the wealth in their possession is lawful until proven otherwise.

However, if it is known that some of a person's wealth is unlawful, it is permissible to accept their gift, though there is no harm in refraining from it due to the reprehensibility of dealing with such a person.

As for the story referred to, there is no knowledge of it.