Here are Six Quran verses on the TOPIC
Quran 23:52-55 ''And verily this Brotherhood of yours is a single Brotherhood, and I am your Lord and Cherisher: therefore fear Me (and no other). But people have cut off their affair (of unity), between them, into sects: each party rejoices in that which is with itself. But leave them in their confused ignorance for a time.Do they think that because We have granted them abundance of wealth and sons.We would hasten them on in every good? Nay, they do not understand.''
3:103 ''And hold fast, all of you together, to the Rope of Allah (i.e. this Quran), and be not divided among yourselves, and remember Allah's Favour on you, for you were enemies one to another but He joined your hearts together, so that, by His Grace, you became brethren (in Islamic Faith), and you were on the brink of a pit of Fire, and He saved you from it. Thus Allah makes His Ayat (proofs, evidences, verses, lessons, signs, revelations, etc.,) clear to you, that you may be guided.''
3:105 ''And do not be like the ones who became divided and differed after the clear proofs had come to them. And those will have a great punishment
3:19 ''Indeed, the religion in the sight of Allah is Islam. And those who were given the Scripture did not differ except after knowledge had come to them - out of jealous animosity between themselves. And whoever disbelieves in the verses of Allah , then indeed, Allah is swift in [taking] account.
45:17''And We gave them clear proofs of the matter [of religion]. And they did not differ except after knowledge had come to them - out of jealous animosity between themselves. Indeed, your Lord will judge between them on the Day of Resurrection concerning that over which they used to differ.''
17:53 ''And say to My slaves (i.e. the true believers of Islamic Monotheism) that they should (only) say those words that are the best. (Because) Shaitan (Satan) verily, sows disagreements among them. Surely, Shaitan (Satan) is to man a plain enemy.''
May Allah swt guide us to be United as he has ordered All of us.