The prophet's (pbuh) saying to Imam Ali (as) "You are the khalifah of every believer after me" through a relaible chain.
It was narrated in the book of the sunnah of Abu Isam on pages 519-520 through a relaible chain reaching to Ibn al-Abbas (ra) that he heard the prophet (pbuh) saying to Imam Ali (as) " Your (Ali) position to me is like the position of Harun to Mosa, except that you are not a prophet, it is not beneficail for me to leave with exception of you being the khalifah of every believer after"
al-Albani declares the above isnaad of the above hadith as hassan (relaible) and says all the narrators in the chain are relaible, he also quote al-Thahabi and al-Hakem declaring the isnaad of the hadith as saheh.
Scans from the book Assunah for ibn Abu Isam, with takhrij of Assunah for al-Albani