The statistic comes from the European Central Bank's study on Cash Use by Household in the Euro Area, published in November 2017.
The report presents estimations of the number and value of cash transactions in all 19 euro area countries in 2016, based on survey result as well as figures on main payments by demographic. The study found that Estonians used cash for 31% of payments in terms of number.
"In 2016 euro area consumers made 1.6 payments per day on average, which equals nearly 11 payments per week. They used cash more frequently than other payment instruments, making on average 1.2 cash payments per day, i.e. almost nine per week." (p. 19)
“In terms of value of payments made at POS, in all countries the share of cash was much lower than in terms of number of payments. In the Benelux countries, France, Estonia and Finland the share ranged from 27% to 33%.” (p. 19)