Noé describes it well here.
The TL;DR is, make something ("Insert Quote/Attestation") similar to the Insert Media dialog, which looks for a certain word/sentence on sister projects like Wikisource, Wikiquote etc., allows to select the snippet where the word/sentence was found, and to have it added to the article as an quote/attestation of use with a link to the whole source document.
Was also suggested during Community Wishlist Survey 2017 and ends up with 32 supports vote.
Yes, this would be great, doing a manual search on Wikisource + copy/paste is very awkward.
For inspiration, there's already a gadget called Quiet Quentin which uses Google Books as citation source. But Wikisource/Wikiquote (maybe with bidirectional linking) would make a lot more sense – the content is directly linkable and the full text is always available.
I'm not sure what the Discovery search team can help with at this point, removing search backlog tag.
@Noe You have just changed the context of the request and I believe that it no longer reflects the initial desire. One would hope that the best solution is directly link and the text as it is at the wiki, ie. a translcusion, or transclusion-like.
If this is a new community desire, then it would be worthwhile linking to that change, as it seems to be your wish rather than the community's.
I have tried to add the original context as this was more than a Wiktionary item. There seems to be a couple of ideas on whether it is a copy/paste or a reference/tranclusion approach. Maybe leave that open for further discussion as both certainly have benefits and downsides.
I didn't realized I changed the direction of this idea. It was already clear when it was proposed to the Community Wishlist Survey in 2016: https://meta.wikimedia.org/wiki/2016_Community_Wishlist_Survey/Categories/Wiktionary#CW2016-R097
In my opinion, there is no need for a transclusion, because texts in Wikisource are stables, but, I agree to let this point open for further discussion.