It has been my experience that most lenders want see that the cash has been in your account for a given period of time (i.e. >30 days). I've run into this several times in that I won't buy a house w/o 20% down to avoid otherwise included the taxes and insurance.

The reason this comes up is because they don't want to risk loaning you a pile of dough based on what appears to be your strong financial position (or ability to pay) only to find that you "borrowed" the money from a rich in-law to qualify. Worse yet you may have taken a huge cash advance or other loan to cover the closing/down and now they are at risk of loosing more.