This page explains how to start and complete the A Gift of Nightshade side quest.
How to Start the Quest
East of Faron Tower, you can find Tuft Mountain, which has a heart-shaped pond on the top. Go there and talk to Wabbin to start the quest.
How to Complete the Quest
You will need a Blue Nightshade for this quest. If you don't have any, you can find some nearby, west of the heart-shaped pond, on the western shore of Yambi Lake.
When you have some Blue Nightshade, talk to Wabbin until he asks if you have some, and say yes. Then agree to give it to the Gerudo who is north of the pond. Go talk to her and give her the Blue Nightshade. After some dialogue, Wabbin gives you a red rupee, and a silver rupee.