Sayyiduna Salman (may Allah be pleased with him) narrated: I said: "O Messenger of Allah! Every prophet has a wasi (one entrusted with his affairs), so who is your wasi?" He (peace and blessings be upon him) remained silent and gave me no answer. Some time later he (peace and blessings be upon him) saw me and said: "O Salman!" I hurried towards him and said: Here I am. He (peace and blessings be upon him) said: "Do you know who the wasi of Musa (peace be upon him) was?" I said: Yes, Yusha' bin Nun. He (peace and blessings be upon him) asked: "Why?" I said: Because he was the most knowledgeable of them. He (peace and blessings be upon him) said: "Indeed, my wasi, the keeper of my secret, the best of those I leave behind me, the one who will fulfil my promise and pay my debt, is Ali bin Abi Talib." Abu al-Qasim (al-Tabarani) said: His (peace and blessings be upon him) saying "my wasi" means that he (peace and blessings be upon him) entrusted him with his family, not with the caliphate; and by his (peace and blessings be upon him) saying "the best of those I leave behind me" is meant the best of his (peace and blessings be upon him) household (Ahl al-Bayt).
Hadith Referenceالمعجم الكبير للطبراني / باب السين / 6038