I’ve heard this story before in talks but only just spotted its source (Jerome).

When the venerable John could no longer walk to the meetings of the Church but was borne thither by his disciples, he always uttered the same address to the Church; he reminded them of that one commandment which he had received from Christ Himself, as comprising all the rest, and forming the distinction of the new covenant, “My little children, love one another.”
When the brethren present, wearied of hearing the same thing so often, asked why he always repeated the same thing, he replied, “Because it is the commandment of the Lord, and if this one thing be attained, it is enough”

[JEROME].