Earliest usage of 力不从心? What's the earliest usage of 力不从心?
The earliest I've seen is:

《後汉书·西域传》
今使者大兵未能得出, 如諸國力不從心, 東西南北自在也。

Anything earlier than 后汉书? 445 is, apparently, the given year for completion of 后汉书.
One of the most useful resources for checking this sort of thing is the huge database from Academia Sinica (中央研究院), 漢籍電子文獻. 

The earliest instance in 漢籍 is just what you cite, 後漢書西域傳, but 漢籍 also lets you look at the context, which can sometimes give a more precise date.  It turns out this line is attributed to 廣武帝, in 廣武二十一年. This would be A.D. 46-7. 

Works such as Shi ji, Han shu, San guo zhi, Hou Han shu, and so on, often incorporate early original documents; if you want to date something, this can be very helpful.

I also can't help feeling that this expression is related to the famous line 七十而從心所欲 in the Analects. 從心 is 從心所欲, right?