عَينًا فيها تُسَمّىٰ سَلسَبيلًا

A spring in it, named Salsabīl.

EXEGESIS

Salsabīl is the name of a fountain in paradise. Ibn al-Aʿrābī states that he never saw this word except in the Quran.^{‎[1]Lisan, 11/344.} Zajjāj explains that this name was chosen for the said fountain because the experience of drinking from it is marked by agreeability and smoothness.^{‎[2]Lisan, 11/344.} Others have said that the fountain earned this name because of the ease of its flow.^{‎[3]Tabari, 29/135.}

[1] Lisan, 11/344.
[2] Lisan, 11/344.
[3] Tabari, 29/135.