The Third Party Rule
It concerns especially the international exchange of information between intelligence services. If the government is interested in cooperation between its intelligence services and intelligence services of foreign countries, it is absolutely necessary to not disclose the source of information and the information itself. Every such intelligence remains the property of the country which provided it and it cannot be shared with anyone else (a third party) without the permission of the originating country. Information is shared with intelligence services of foreign countries provided that the conditions stipulated for "intelligence sharing" are met. The rule of reciprocity is expected here.
Source: Petr Zeman - lectures, trainings and public speeches