“Many a desperate dash down the base lines was made and many a remarkable ‘belly slide” was attempted by men who have since become dignified in their manner and prominent in the business and professional worlds.” The Sioux City Times, February 28, 1901, p13
NOTE (Ken Liss): There are also earlier examples of it being called a “stomach slide.”
“…. he [Jim Fogarty] will never go begging for a position in any of the first-class clubs while his strong right arm retains its cunning, and his patent stomach-slide does not fail him.” San Francisco Examiner November 19, 1887, p6
This illustration, which originally appeared in the New York World, was in an article on different styles of sliding that appeared in several newspapers in 1888.
Previous earliest use (Dickson Baseball Dictionary, 3rd edition, 2009):
1944. “Eddie [Mota] lost his footing and did a beautiful belly-slide across the infield” (Waukesha [Wis.] Freeman, Sept. 20; Peter Morris).