Cicero, Letters to Friends 10.31.6
quod familiarem meum tuorum numero habes, opinione tua mihi gratius est;
invideo illi tamen, quod ambulat et iocatur tecum.
That you consider my friend [Gallus] one of your own is more gratifying to me than you can imagine. But I am envious, because he is able to walk and joke with you.