The Empire Strikes Back - Teaser Trailer
https://www.youtube.com/watch?v=S_4QAVa9K_k - from the THX1968 YouTube channel (1:25 mins long)

This was the highest quality version I could find online, but keep in mind that he image has been cropped.

Backup from the Official Star Wars YouTube channel. The quality on this version is much worse, but it does have the proper aspect ratio.

The blurb:
“The original teaser trailer for The Empire Strikes Back, notable for solely featuring concept art by Ralph McQuarrie.”