Generally, in formal writing, one would "cite the source" of the quote.

Truss says this usage is no longer considered proper in formal writing.

Written history is only approximately 5000 years old (the age of formal writing).

She was described to have been, unlike even most noble women of the time, well-learned in literature and formal writing.

On the other hand, formal writings were still mostly done in classical Chinese.

Draft an offer for the house in formal writing.

The modern literary style is generally used in formal writing and speech.

"They should know where to draw the line between formal writing and conversational writing."

It's hard enough to teach them the art of formal writing.

Slang words are not appropriate in formal writing or speech.