This general material had to divide up into its constituent details, and then, first of all, there was the general basic foundation, which is called Atzilut with its three garments. Afterwards, control passed successively to each garment by itself, and they were called the three worlds of Beriyah, Yetzirah and Asiyah. For Atzilut, as arranged with its proper repairs and adornments, is the root of everything, while Beriyah, Yetzirah and Asiyah serve to bring forth the offshoots of Atzilut. Each one individually brings forth the offshoot suited to it, and all of them are merely offshoots of what already exists in Atzilut.
Story of Atzilut
Kalach Pitchei Chokhmah 40:2