Two bodies of flowing water: One palatable and sweet and the other salt and bitter they don't mix.
He has let free the two bodies of flowing water meeting together: Between them is a Barrier which they do not transgress: [Qur’an 55:19-20, Translated A.Yusuf Ali]
Or who has made the earth firm to live in; made rivers in its midst; set thereon mountains immovable; and made a separating bar between the two bodies of flowing water? (Can there be another) god besides Allah? Nay most of them know not. [Qur’an 27:61, Translated A.Yusuf Ali]
It is He Who has let free the two bodies of flowing water: one palatable and sweet and the other salt and bitter; yet has He made a barrier between them a partition that is forbidden to be passed. [Qur’an 25:53, Translated A.Yusuf Ali]