Many sources state it. There were rumors early on perpetuated by IGN that it was 64MB, but it has since been shown to be incorrect via people who disassembled the system manually to discover the specs. Either Nintendo upped the memory before release or IGN was as usually full of crap (could be either one but I wouldn't put it past IGN to lie). Anyways, many sources but here's a couple-
http://www.nintendoworldreport.com/news/25900/3ds-memory-total-revealed
http://3dbrew.org/wiki/Hardware
http://www.neowin.net/news/factory-worker-pinches-nintendo-3ds-shows-it-off-to-the-world
The 3dbrew article shows all the 3DS' specs, not just the memory. Even the brand and model numbers of the ram are known. 3DS has two Fujitsu MB82M8080-07L ram modules 64MB in size each, 128MB in total. It's a type of memory known as FCRAM, which is a relatively fast variety of memory compared to the likes of Wii and previous consoles (capable of bandwidths of 3.2GB/s). 32MB of that is reserved for the Operating System running in the background, leaving 96MB for games. The 96MB number was discovered from a photo a factory worker took of the system. They stole the handheld off an assembly line before the system released to the public and he took some photos of the device. One of the pictures showed the system while it was running some sort of debug program and it listed 96MB usable memory. As we know the device has 128MB of system ram based on teardowns of the 3DS, it makes sense that the OS would reserve 32MB.