Reinhard Müller found this method of palming a card from the center in F. W. Conradi's in Der Moderne Kartenkünstler, 1896, p. 13. Conradi claimed the sleight as original with him.
It appeared in English as part of Adrian Plate's excelsior change in August Roterberg's New Era Card Tricks, 1897, p. 22. This was a steal of the bottom card of a double, rather than one from the center of a deck, but the sleights are related.
A closer precursor to the now-traditional side steal appears in Mahatma, Vol. 5 No. 2, Aug. 1901, p. 495. This is by Hal Merton (stage name of Walter G. Peterkin) and was published eight years prior to T. Nelson Downs and John Northern Hilliard's The Art Magic, 1909, p. 143, where the first description of the side steal is usually cited. Merton used his left thumb to push over the top card of the lower packet as the right hand moved to replace the upper packet. This delivered the card into position to be palmed, identically to the standard side-steal handling. A closer handling to Downs's—using the left fingers beneath the card to be stolen, rather than the thumb above—appeared in a later issue of Mahatma. It was published without credit or byline in Vol. 8 No. 2, Aug. 1904, p. 17.