1946: "Loophole", a short story by Arthur C. Clarke, first published in Astounding Science Fiction, April 1946, available at the Internet Archive.
From Wikipedia summary:
The story begins in the form of correspondence between the President of Mars and the Secretary of the Martian Council of Scientists, regarding the discovery of atomic power (in the form of atomic bombs) by humans. They are concerned that once humanity's current war is over (they have been monitoring Earth's broadcasts), humans will use atomic power and rockets to breach interplanetary space and pose a threat to Mars. A remote monitoring station is set up by Mars on the Moon to monitor Earth's progress. Finally they send a fleet of 19 battleships along with a warning to Earth that one city will be destroyed every time a rocket leaves Earth's atmosphere. Earth agrees to stop experimenting with rockets when they realize their broadcasts are being intercepted. Ten years pass without any further rocket experimentation, while the Martians plan for the extermination of the human race, believing that Earth will always be a threat to them.
Excerpt from story:
From: Secretary, Council of Scientists.
The facts are as follows. Some months ago our instruments detected intense neutron emission from Earth, but an analysis of radio programs gave no explanation at
the time. Three days ago a second emission occurred and soon afterwards all radio transmissions from Earth announced that atomic bombs were in use in the current war. The
translators have not completed their interpretation, but it appears that the bombs are of considerable power. Two have so far been used. Some details of their construction have been released, but the elements concerned have not yet been identified. A fuller, report will be forwarded as soon as possible. For the moment all that is certain is that the inhabitants of Earth have liberated atomic power, so far only explosively.