On July 21, 1748, Benjamin Franklin wrote “Advice to a Young Tradesman,” addressed to “my Friend A. B.” Early in the piece he writes: “Remember that Time is Money.” A man who can earn ten shillings a day and “sits idle one half of that Day” has lost more than the sixpence he spent on amusement. “He has really spent or rather thrown away Five Shillings besides.”
The phrase itself is older. It was in print by 1719.