This Is My Faith - Al-fatlawi, Ali - الصفحة ٢٧٤ - the Scripture or Election?
(And warn your nearest relations)٢٦;٢١٤.
So, the Prophet (PBUH) gathered his tribe of near kindred, fed them and lastly asked them if any of them is willing to be his minister, trustee and successor after him. Everybody refrained except for Ali (PBUH), so the Prophet (PBUH) said: “This is my brother, trustee and successor after me; listen to him and obey him”.
The second case was when the Prophet (PBUH) left him in Medina to go for Tabouk conquest. So, Ali (PBUH) wondered why the Prophet (PBUH) left him behind with women and children; the Prophet (PBUH) replied: “You are of the same status to me as Aaron to Moses except there is no prophet after me”. You know better that Aaron was the successor of Moses (PBUH).
The third evidence is related to the well-known Ghadir Day and the speech the Prophet Mohammad (PBUH) gave in which he said: “Whoever I am his master, then this is Ali his master. O Lord! Support whoever supports him, show enmity towards whoever shows enmity towards him and, let triumph whoever stands up for him, forsake whoever forsakes him and let the truth follow him wherever he turns”.