Lovely "surprise attack" opening! Not commonly used in classical games.
The idea of sacrificing a pawn with ...b5 and ...a6 is quite old. Karel Opočenský applied the idea against, among others, Gideon Ståhlberg at Poděbrady 1936, Paul Keres at Pärnu 1937, and Erich Eliskases at Prague 1937. Later Mark Taimanov–David Bronstein, Candidates Tournament, Zürich 1953, drew attention. Most of these games began as a King's Indian, with Black only later playing ...c5 and ...b5. Possibly the first to use the now-standard move order 1.d4 Nf6 2.c4 c5 3.d5 b5 was Thorvaldsson-Vaitonis, Munich Olympiad 1936.
The original name of the opening was the Volga Gambit, named after the Volga River because of an article about 3...b5!? by B. Argunow written in Kuibyshev (Samara since 1991), Russia, that was published in the second issue of 1946 of the magazine Schachmaty in USSR. The term is still widely used in Russian literature.
Beginning in the late 1960s, this opening idea was also promoted by Pal Benko, a Hungarian-American Grandmaster, who provided many new suggestions and published his book The Benko Gambit in 1974. The name Benko Gambit stuck and is particularly used in English-speaking countries.
Though "Volga Gambit" originally referred solely to the move 3...b5 (sometimes followed by an early ...e6), while Benko himself analyzed in his Batsford treatise solely what is now the main line, 3...b5 4.cxb5 a6, both the terms Benko Gambit and Volga Gambit are now used interchangeably or concurrently (for example, Volga-Benko Gambit).