Sayyiduna Jubayr bin Mut'im (may Allah be pleased with him) heard the Prophet (peace and blessings be upon him), when he (peace and blessings be upon him) gave the key of the Ka'bah back to Uthman bin Talhah, say to him: "Here, take it, and keep it hidden." The narrator said: It is for this reason that the key is kept hidden.
Hadith Referenceالمعجم الكبير للطبراني / باب الجيم / 1523