From Knuth vol 2 section 4.1: Our decimal notation...was developed first in India within the Hindu culture...The earliest known Hindu manuscripts that show dec

From Knuth vol 2 section 4.1: Our decimal notation...was developed first in India within the Hindu culture...The earliest known Hindu manuscripts that show decimal notation have numbers written backwards (with the most significant digit at the right), but soon it became standard to put the most significant digit on the left." [later] "It is interesting to note that the left-to-right order of writing numbers was unchanged during [translation from Hindu to Arabic to Latin], although Arabic is written from right to left while Hindu and Latin scholars generally wrote from left to right. A detailed account of the subsequent propagation of decimal numeration and arithmetic into all parts of Europe during the period 1200-1600 has been given by David Eugene Smith in his History of Mathematics I, chapters 6 and 8."