He never announced it publicly and he had access to decent enough system(s) that allowed him to mine bitcoin in very first days.
My first Bitcoins were mined using a Sempron 1300 Mhz, 1024 Mb RAM... Bitcoin has no hardware requirements on its own, its the miners competing with each other that causes difficulty to grow.
pooya87 is referring to the first few blocks that are basically guaranteed to have been mined by satoshi himself. As the difficulty is a function of the total network hashrate and the block time, by assuming satoshi had the total network hashrate in those first days, we can calculate from the (known) early blocks difficulty target and block time, how much computing power he could access.
There is a thread about it here:
Does it sound right? Estimating the hashrate of CPU likely used to Mine Genesis