It's in the skill description. There you go:
Special: You may voluntarily add +10 to the indicated DC to craft an item. This allows you to create the item more quickly (since you'll be multiplying this higher DC by your Craft check result to determine progress). You must decide whether to increase the DC before you make each weekly or daily check.
Edit: Ok, the thing pointed out by DM Blake seems to be more what you were thinking of...
Magic Item Creation in the core book. This paragraph. I bolded the pertinent part:
"The creator also needs a fairly quiet, comfortable, and well-lit place in which to work. Any place suitable for preparing spells is suitable for making items. Creating an item requires 8 hours of work per 1,000 gp in the item's base price (or fraction thereof), with a minimum of at least 8 hours. Potions and scrolls are an exception to this rule; they can take as little as 2 hours to create (if their base price is 250 gp or less). Scrolls and potions whose base price is more than 250 gp, but less than 1,000 gp, take 8 hours to create, just like any other magic item. The character must spend the gold at the beginning of the construction process. Regardless of the time needed for construction, a caster can create no more than one magic item per day. This process can be accelerated to 4 hours of work per 1,000 gp in the item's base price (or fraction thereof) by increasing the DC to create the item by 5."
The first part of the very next paragraph says the following:
"The caster can work for up to 8 hours each day. He cannot rush the process by working longer each day"
Some DMs interpret that first sentence to mean you can work 8 hours no matter what, which means you can do two of these 4-hour shifts each day, which literally means that you do twice as much work each day, reducing the total crafting time to half the number of days (same number of hours, half the days, +5 penalty to the craft DC).
But other DMs interpret the second sentence to mean that if you accelerate the time to 4 hours, that is still all the work you can do that day because doing more would mean you are "rushing the process by working longer that day". In this case, you're working longer than the allowed 4 hours, and since rushing the process is clearly forbidden by RAW, you can't do it. In other words, reducing the 8 hours to 4 hours doesn't change the amount of magical energy you can imbue into an item, all it does is free up the second half of your workday to do something else.
This is a debatable topic.
Check with your own DM to see which interpretation he prefers, or if you're the DM, you decide which one you prefer.
Yes.prd crafting