Apple iPhone iOS 8.1 - Manual (Page 77)

Chapter 9 Messages 77
Messages settings
Go to Settings > Messages, where you can:
Notify others when you’ve read their messages
Specify phone numbers, Apple IDs, and email addresses to use with Messages
SMS and MMS options
Block unwanted messages
Set how long to keep messages
Manage the expiration of audio messages and video messages created within Messages (audio
or video attachments created outside of Messages are kept until you delete them manually)
See Do Not Disturb on page 34.
Set the alert sound for incoming text messages. See Sounds and silence on page 34.