Yes, this is commandment of Allah in quran. And not in one place, we find this in enamores places in quran and hadees. Collecting them, we can conclude it is obligatory.
Though, for a believer, commandment of Allah is enough. If Allah has commanded us, so we should do as it. Without questioning if it's obligatory or not. Because if your lord say so, just do it.
Now, let's discuss some references of this matter:
"The Believers, men and women, are protectors one of another: they
enjoin what is just, and forbid what is evil: they observe regular
prayers, practice regular charity, and obey Allah and His
Messenger..." (at-Tawba, 9/71)
In this verse, it is clearly mentioned that believers does so. What if someone does not do so? well, he is not complete(or perfect) believer... this is conclusion from this ayah.
As an example, just like properties of a specific element in chemistry, if we know a specific element reacts in a specific way with certain element, and if it does not do so, we will investigate if we have that element pure? Because it is not doing what it is expected to do. Similarly, here Allah describes the property of believers, so if a believer is not doing so, we cannot say he is a perfect believer. Because this property is defined by Allah not by any human being.
About believers Allah say in quran:
The Messenger has believed in what was revealed to him from his Lord,
and [so have] the believers. All of them have believed in Allah and
His angels and His books and His messengers, [saying], "We make no
distinction between any of His messengers." And they say, "We hear and
we obey. [We seek] Your forgiveness, our Lord, and to You is the
[final] destination." 2:285
So, obeying what they know is from Allah is also a property Allah has described for true believers.
A few ahadees for the matter: (just to shed some light of importance of this)
You see the believers as regards their being merciful among themselves and showing love among themselves and being kind,
resembling one body, so that, if any part of the body is not well then
the whole body shares the sleeplessness and fever with it." (see
Bukhari, Salat, 88)
"Help your brother, whether he is an oppressor or he is an oppressed
one." When he was asked how to help whenhe is an oppressor, the
Prophet said,"By preventing him from oppressing others. This is how to
help him then." (Bukhari, Mazalim, 4; Muslim, Birr, 62)
"A Muslim has six rights over another Muslim. When he meets a Muslim,
he greets him; when a Muslim invites him, he accepts his invitation;
when a Muslim sneezes and says, 'alhamdulillah (praise be to Allah)',
he says, 'yarhamukallah (may Allah have mercy on you)'; when a Muslim
becomes ill, he visits him; when a Muslim dies, he takes part in his
funeral; he loves for his Muslim brother what he loves for himself."
(Darimi, Istidhan: 5; Ibn Majah, Janaiz: 43)
May Allah unite muslims again as his commmandment, by his will (انشا اللہ)
Allah knows best
