An-Nahl - 16:93

وَلَوْ شَاء اللّهُ لَجَعَلَكُمْ أُمَّةً وَاحِدَةً وَلكِن يُضِلُّ مَن يَشَاء وَيَهْدِي مَن يَشَاء وَلَتُسْأَلُنَّ عَمَّا كُنتُمْ تَعْمَلُونَ
[16:93]
And if Allah please He would certainly make you a single nation, but He causes to err whom He pleases and guides whom He pleases; and most certainly you will be questioned as to what you did.
If Allah so willed, He could make you all one people: But He leaves straying whom He pleases, and He guides whom He pleases: but ye shall certainly be called to account for all your actions.
Had Allah willed He could have made you (all) one nation, but He sendeth whom He will astray and guideth whom He will, and ye will indeed be asked of what ye used to do.