After 9/11 and the enhancements of the Money Laundering Control Act of 1986 (MLCA) via the Patriot Act (October 2001), banks now require thorough documentation on any large deposits or gift funds. Despite what people are saying here, underwriters who don't ask for a paper trail are playing with fire and will eventually get burned. Consumers should go on the assumption they'll have to provide a paper trail for any funds gifted and be prepared if the donor account has other large transactions (non-payroll direct deposit), they may be asked about those funds, up to and including documenting the source of that transaction and on-and-on until the trail stops or the funds can't be sourced. The underwriters who aren't asking for the full paper trail aren't doing their due diligence to address the government's money laundering concerns and as a result, may be out of a job one day.
Assume the worst, plan to document everything.