Sunan an-Nasa'i, Volume 4, Book 30, Hadith 3695 (DarusSalam) ⓘ
It was narrated that Sa'd bin 'Ubadah said: "I said: 'O Messenger of Allah, what kind of charity is best?' He said: 'Providing drinking water.'"
أَخْبَرَنَا أَبُو عَمَّارٍ الْحُسَيْنُ بْنُ حُرَيْثٍ، عَنْ وَكِيعٍ، عَنْ هِشَامٍ، عَنْ قَتَادَةَ، عَنْ سَعِيدِ بْنِ الْمُسَيَّبِ، عَنْ سَعْدِ بْنِ عُبَادَةَ، قَالَ قُلْتُ يَا رَسُولَ اللَّهِ أَىُّ الصَّدَقَةِ أَفْضَلُ قَالَ " سَقْىُ الْمَاءِ " .
also referenced under: Hadith 3665 (Reference) & Book 30, Hadith 55 (In-Book)