4th level Druid spell, PHB 59.

The rules are not clear whether the spell must be cast on the druid (and the 20' diameter sphere follows the druid), or if the spell must be cast on the precise location the druid is standing (and the 20' diameter sphere does not follow the druid). Therefore, follow the precedent set by Silence, 15' radius, and allow either at the druid's option at casting time.