Adam Clarke on John 15:12

That ye love one another - See on Joh 13:34 (note). So deeply was thus commandment engraved on the heart of this evangelist that St. Jerome says, lib. iii. c. 6, Com. ad Galat., that in his extreme old age, when he used to be carried to the public assemblies of the believers, his constant saying was, Little children, love one another. His disciples, wearied at last with the constant repetition of the same words, asked him, Why he constantly said the same thing? "Because (said he) it is the commandment of the Lord, and the observation of it alone is sufficient." Quia praeceptum Domini est, et, si solum fiat, sufficit.