“In the morning, I shall hand over this flag to that person who ALLAH:subhanahu: and His Prophet:saw2: keep friend.”
These were the words of Prophet Muhammad 
on the day of Ghazwatul Khaiber.
According to the narrations by Companions(r.a): Suddenly Syedana Ali Al Murtaza:razi: appeared (and in some narrations, it is said that Hazrat Ali Murtaza 
was called) and was given that flag by Prophet Muhammad:saw2:
(Saheeh Bukhari)
Syedana Ali Al Murtaza:razi: is one of the Companions among Ashra-e-Mubashira (those who were given the glad tidings of paradise by the prophet in this life).
Khalifa-e-Rashid Ali was born 29 years after the incident of Aa’mul-Feel, on Friday 13th Rajab. His patronymic name was Abul Hasan, mother’s name was Fatima Bint-e-Asad and father’s name was Abu Talib. He was the cousin of Prophet Muhammad:saw2: and belonged to the respectful family of Banu Hashim. He was the youngest one to embrace Islam at the age of 13 - the first child to come into the fold of Islam.
He was one of the best and beloved Companions of Prophet Muhammad(SAW), who at the time of migration, slept on the bed of Prophet Muhammad(SAW) when Prophet Muhammad(SAW) left with Syedana Abu Bakr(RA) for Madinah at night.
He was given the titles of “Al-Murtaza” (The Brave) and “Asadullah” (Lion of Allah) by Prophet Muhammad(SAW). The memorable event, breaking the door of Khaibar also took place by the hands of Syedana Ali(RA). Apart from cousin, he was also the son-in-law of Prophet(SAW). He was married to the most beloved daughter of Prophet(SAW), Hazrat Fatima Az Zahra:razi2:.
He passed his great Caliphate of 6 years after the martyrdom of Syedana Usman(RA).
Unfortunately, many problems and propagandas were raised during his Caliphate but He faced them with the open heart. He interrogated for the killers of Syedana Usman(RA) but didn’t find any of them.
In 40 Hijri, at the age of 63, during the month of Ramadaan when he was offering Faj’r prayer, a Jew named “Ibn-e-Maljam” inserted a dagger in his back. After three days of the incident, sydana Ali (RA) left this world on Ramadaan 17th.
There are some narrations of him being martyred on either 19th, 21st, or 30th of Ramadaan as well.
Rasoolulah had said: *“No doubt, Ali is from Me and I am from Ali, and Ali will be the Saint of every Momin after Me, and will be love by ever momin, and no momin will hate him.” - *(Tirmidhi)
In a narration in Saheeh Muslim, Syedana Ali(RA) is quoted to say that Rasoolulah(SAW) promised me that no one except a momin can love me and no one except a hypocrite can hate me.
Contents taken from multiple sources and are paraphrased.
for the information.