250 kts in the USA?

[xray one]

Sorry if it's been covered before.

I can't lay my hands on my FAR AIM at the moment and would like to know the following:

Do you need to be at 250 Kts at 10,000' or can you start to reduce to 250 Kts once passing 10,000'?

If it's not in FAR AIM, where is it officially written down?

Thanks in advance