When is the US Senate required to pass bills unanimously?

According to the BBC News website, the US senator Rand Paul has managed to delay approving financial aid package to Ukraine:

Paul, who has historically opposed spending on foreign aid, refused to back the proposal which needed unanimous support.

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Regardless whether we agree or not, it seems strange to me that the US Senate needs to pass it unanimously. Is it always the case? Why isn't normal majority required? Is it a special case? If so the question is:

When is the US Senate required to pass bills unanimously?