Sourcing isn’t needed whenever you download from sites like Pixabay, pexel etc. It's not mandatory. It's already written on those websites - crediting is not required! This is the universal rules, applicable for all sites. Now if anyone wants to provide credit to Pixabay, that depends on content creators. But if you don't that's not plagiarism. Thanks for your concern though.
I already wrote an article on this topic. You may check out - Top 7 websites for copyright free images
I hope you get my point now.
(Note- as you already tagged hivewatchers I would love to hear his comment on this matter.)
Speical tag - 
RE: Be a Content Writer and Earn Passive Income