Here are two different stories from magazines dated August 1935. I have no idea which one was published first.
1935: "The Man Who Met Himself", a short story by Ralph Milne Farley (pseudonym of Roger Sherman Hoar); first published in the August, 1935 issue of Top-Notch magazine; reprinted in Farley's 1950 time-travel collection The Omnibus of Time. There is a brief description at the Internet Time Travel Dataase.
It is the story of Dick Withrick, a Boston stockbroker on a safari, who finds a mysterious time machine in a Cambodian jungle:
To Withrick's astonished gaze, there was revealed a metal framework of machinery. The metal was uncorroded and bright, as though freshly polished. It glittered in the afternoon sunlight.
And it was exquisitely, delicately made. There was ivory in it, polished ivory; and gleaming brass, and some silvery metal resembling chromium. A large part was made out of some transparent crystalline substance, twisted incongruously.
The general impression which the contrivance gave was that of unreality. The right-angles, at which various bars joined each other, did not seem to be quite ninety degrees. The perspective was distinctly off; for regardless from which side one viewed it, the more distant side always seemed to be the larger, rather than the smaller as it ought. But perhaps this phenomenon was due to the waving heat of the humid jungle.
In the center of this four-foot-square insubstantial framework, was the one touch of substantial reality in the whole contrivance: a crude stamped-metal seat, evidently taken from a tractor. In front of the seat was an instrument-board, with a series of four dials, and what were obviously two control-levers. And, unless Withrick's imagination was playing him tricks, he could almost imagine the vague outline of a man on the tractor-seat.
When one of the natives tells him that it's a time machine, Withrick immediately thinks of using it to beat the market:
"Of course it's all bosh!" he declared aloud. And yet wouldn't it be wonderful if he could travel a few years into the future, learn the course of the stock-market, and then return to the present, and play the market on that information!
Instead, Withrick travels backward in time from 1935 to 1925 and makes his killing there, in a sort of partnership with the local Abbot:
The time-traveling was true! Withrick had actually traveled backward ten years through time! For a moment he was stunned — desolated. But then the possibilities of the situation began to dawn on him!
"I have been here before — I know the grass beyond the door, the sweet keen smell," he quoted, jubilantly. For he had already passed through the years from 1925 to 1935 once before, as stock-broker's clerk, then stock-broker, and finally member of one of the leading firms on the Boston 'Change. And now these very same years lay ahead of him again. His retentive memory contained a complete chart of the week-to-week movement of the principal stocks for that entire period. Now, living through it again, with that advance information at his disposal, he could make himself a multimillionaire! But he must keep under cover while doing it. What better place than here in this secluded Buddhist monastery!
He explains his plan to the Abbot:
"You are certain that this play-the-market will succeed?"
"It can't fail, Your Grace. The future, for the next nine and a half years, has already happened for me. I've been through it once. I remember every detail of it. — And yet. And yet. Say, I'm not so sure. The market is always affected by any outside interest getting into it and playing it for big stakes. You weren't in the market, the time that I went through the years from 1925 to 1935 before. And so I'm afraid that your getting into the market this time will change things; will cause the market to react differently."
But the wise old Buddhist prelate shook his shaven head. "No," he said. "Nothing that we, or any others do, can change the events of those ten years. What is written, is written. The wheel of life turns. Shiva breathes. And so, although I not to quite understand your market yet, I do know that what is written, is written."
1935: "The Morrison Monument", a novelette by Murray Leinster, first published in Argosy, August 19, 1935; reprinted in Avon Science Fiction Reader #1, 1951, available at the Internet Archive.
"I set this lever on the dial," said Morrison anxiously, "and press the
other lever. The machine snaps ahead in time to the point the first lever is
set for. It stops in that moment of the future. The camera shutter then clicks over, and in doing so actuates the return mechanism. Then the whole
machine comes back to the exact instant it started from, so that—well—practically it's been there all the time. I'll show you."
[. . . .]
Morrison bad been very obedient in the taxicab, and he continued to be
obedient in the days that followed. Craig schemed feverishly and demanded
photographs of most improbable objects, as they would look at most improbable times. Racing sheets—photographed as they would later be hung on newsstands beneath the Sixth Avenue Elevated—to tell him the results of horse races before they were run. Headlines in the more conservative journals to tell him of the two teams approaching the World's Series in baseball. Other headlines to tell him that the series would run a full seven games; and who would win the series; who would win each game; and the score inning by inning.
Craig inquired feverishly into the policy and number games of Harlem. He sent Morrison into dingy places to photograph—days in the future—the notices which would tell him the winning numbers before they won. And as the reams of plates yielded more advance information and his memoranda became systematized, Craig's hopes became fiercer and more desperately near to belief.
From the very beginning he laid out Morrison's money according to what the Financial Daily would say much later on. Then he waited in anguished impatience for the advance photographs to be verified like the first ones. But while waiting he found that he had known accurately events twenty-four hours ahead, then forty-eight, then seventy-two hours before they happened. He knew that Lucky Lady would win at Havre de Grace and what odds she would pay before she even went to the post. He knew that 792 would pay off in Honest Joe Griffin's number game in Harlem, long before Honest Joe knew it himself.
He knew that St. Louis would nose out the Giants for leadership of the National League while the rest of the world speculated feverishly. He knew in advance the result of the America's Cup races, down to the last protest flag. And these things, as days passed, turned out as he anticipated them.
Down to the smallest items, the newspapers photographed by Morrison's time-camera even a week in the future were identical with printed copies when their presses finally spewed them forth. Races, numbers, stocks—the sequence of confirmations was irresistible.
Craig began to use his knowledge of the future on his own account. Through betting commissioners he wagered on the results at Detroit and in Kentucky. In person he put up stakes on such diverse items as the last three figures in clearance house reports—and Honest Joe Griffin lost heavily on this; on the first football games of the season; and on the Hauptmann case developments. And he won.
(Other stories suggested in now-deleted answers: "The Immortality of Alan Whidden" (1942), also by Farley, and "Sunspot Purge" (1940) by Clifford D. Simak.)