عَيْنًا فِيهَا تُسَمَّىٰ سَلْسَبِيلًا
'Aynan feeha tusammaa salsabeelaa
from a spring there called Salsabil.
Salsabīl appears once in the whole Qur'an, and the commentators divided over it: some read it as the proper name of a particular spring, others as a description from a root suggesting water that runs smooth and goes down easily. Either reading lands in the same place in this surah, where the people being served are the ones who earlier gave away food they wanted themselves, to a captive and an orphan, and asked for nothing back.