瑜伽論記 7.52

7.52

如是勤修數息念者，十十是百數以為一乃至滿十，何故從一數俱至十，由此以十為一算數，於其中間心無散亂，齊此名已串修習。

Regarding 'one who diligently cultivates this mindfulness of counting breaths counts ten tens as one hundred, taking it as one, up to the completion of ten': why does one count together from one to ten? Because by taking ten as one count, without the mind becoming distracted in between, this is designated as having become thoroughly practiced.