The first endowment in Islam was the endowment of Umar. The Messenger of Allah ﷺ said to him, "Preserve the principal and dedicate its fruits as charity."
حدثنا حماد أخبرنا عبد الله عن نافع
أول صدقة كانت في الإسلام صدقة عمر فقال له رسول الله ﷺ احبس أصولها وسبل ثمرتها
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Its isnad is sahih, and it is an abridgment of 5947 and 6078.