There was a water that he soaked his eyes in. It was when he was pretending to be a foundation establishment cultivator and infiltrated the Drifting Cloud Sect, there was a trial he partook in
To clarify, he took a root portion of the tree that produces the sap (that is used for the water). The tree is a heavenly wood equal to his bamboo. After he broke through and became an elder of that sect, he repeatedly rinsed his eyes many times to get to that point. A normal foundation establishment cultivator uses one dose (granted up to 10 people every few decades). He used hundreds.