Open WhatsApp
Tap the three dots menu on Android or Settings on iPhone
Go to Settings
Tap Storage and data
Under Media auto-download, tap When using mobile data
Uncheck Photos, Audio, Videos, and Documents
Tap When connected on Wi-Fi
Uncheck Photos, Audio, Videos, and Documents
Tap When roaming
Uncheck Photos, Audio, Videos, and Documents
On iPhone, also go to Chats
Turn off Save to Camera Roll if needed