“Chicks Cop by Rally Put Over in Final Round. Go into Ninth Two Runs Behind; Get to Walk Up; With Ducks on Pond Green Drives ’em Homeward.” (Headline). Chickasha (OK) Daily Express, June 7, 1921, p1
Previous earliest use (Dickson Baseball Dictionary, 3rd edition, 2009):
1ST USE. 1939. “[Joe DiMaggio’s] runs batted in record would indicate he doesn’t hit when there’s ‘ducks on the pond’” (San Francisco News, Aug. 5, 1939, p. 13; Peter Tamony).