Attribution is normally required when using someone else's copyrighted work.
Creative Commons (CC)
Creative commons recommends the TASL approach:
T = Title
A = Author
S = Source
L = License
Examples of CC Attribution

Breakdown:
T = It is the beautiful Egypt
A = Karim Essm
S = It is the beautiful Egypt (link to original Wikimedia Commons page)
L = Creative Commons Attribution-Share Alike 4.0 International (link to license deed)
More examples of TASL at the Creative Commons wiki here: https://wiki.creativecommons.org/wiki/best_practices_for_attribution
Others
Databases sometimes provide their own attribution guidelines e.g. Freesound.