The Ansaar literally means the supporters of Madeenah, were a gift of Allahسبحانه و تعالى to Islam. Their sacrifices were immense and their faith stunning. They believed in Islam as soon as they heard Mus’ab ibn 'Umayr رضي الله عنهrecite the words of Allah سبحانه و تعالىto them, and finally invited the Messenger of Allah ﷺ to move to their township.
The Ansaar knew how to keep their word. They surprised any estimations of human sacrifice the world had known by how readily they embraced their penniless refugee brothers—the Muhajiroon—and shared with them all their wealth and property. They are the heroes who not only followed the Messenger of Allah ﷺbut also supported him with their wealth and their blood. The Sahabah رضي الله عنهmainly consisted of two groups: the Muhajireen and the Ansar. The Ansar were the people from Madinah who gave Nusrah (material support) and the bayah (pledge of allegiance) to the Prophet ﷺ
And the first forerunners [in the faith] among the Muhajireen and the Ansar and those who followed them with good conduct - Allah سبحانه و تعالىis pleased with them and they are pleased with Him, and He has prepared for them gardens beneath which rivers flow, wherein they will abide forever. That is the great attainment. Qur’an.Surah At-Tawba 9:100. 
Narrated Ghailan bin Jarir رضي الله عنه: I asked Anas رضي الله عنه, "Tell me about the name 'Al-Ansar.; Did you call yourselves by it or did Allah سبحانه و تعالىcall you by it?" He said, "Allah سبحانه و تعالىcalled us by it." We used to visit Anas (at Basra) and he used to narrate to us the virtues and deeds of the Ansar, and he used to address me or a person from the tribe of Al-Azd and say, "Your tribe did so-and-so on such-and-such a day." Sahih al-Bukhari 3776 (Vol. 5, Book 58, Hadith 120)
Narrated Al-Bara رضي الله عنه: I heard the Prophet ﷺsaying , "None loves the Ansar but a believer, and none hates them but a hypocrite. So Allah سبحانه و تعالىwill love him who loves them, and He will hate him who hates them." Sahih al-Bukhari 3783 (Vol. 5, Book 58, Hadith 127), Muslim, 75
Narrated Anas bin Malik رضي الله عنه: The Prophet ﷺsaid, "The sign of Belief is to love the Ansar, and the sign of hypocrisy is to hate the Ansar." Sahih al-Bukhari 3784 (Vol. 5, Book 58, Hadith 128) 
The sacrifice of the Ansaar and their preference for their immigrant brothers at their own expense won them applause even from the Lord of seven heavens, and Allah سبحانه و تعالى, Said what means: "And [also for] those who were settled in al-Madeenah and [adopted] the faith before them. They love those who emigrated to them and find not any want in their breasts of what the emigrants were given but give [them] preference over themselves, even though they are in privation. And whoever is protected from the stinginess of his soul – it is those who will be the successful." Qur’an.Surah Al-Hashr 59:9. 
Allah سبحانه و تعالىmentions the types of believers, dividing them into the Muhajirin, who left their homes and estates, emigrating to give support to Allah سبحانه و تعالىand His Messenger ﷺto establish His religion. They gave up their wealth and themselves in this cause. There are also the Ansar, the Muslims of Al-Madinah, who gave asylum to their Muhajirin brethren in their own homes and comforted them with their wealth. They also gave aid to Allah سبحانه و تعالىand His Messenger ﷺby fighting alongside the Muhajirun. The Muhajirun and Al-Ansar are the supporters of each other; Allah سبحانه و تعالىpraised the Muhajirin and the Ansar in several Ayat of His Book and His Messenger ﷺas mentioned in Qur’an.Surah Taubah 9:100, 9:117 
Shaykh al-Islam Ibn Taymiyah رحمت اللهsaid:
If a person slanders them in a way that does not impugn their good character or religious commitment, such as describing one of them as being stingy or cowardly or lacking in knowledge or not being an ascetic and so on, then he deserves to be rebuked and disciplined, but we do not rule him to be a kaafir because of that. This is how the words of those who were not regarded as kaafirs by the scholars are to be understood.