Speak against nobody, and do not slander your mother's son. Who are you that judgest the servant of another? To his own lord he stands or falls; yea, he shall be made to stand, for the Lord has power to make him stand.
Source, in full
Letter 22, p.37; as qtd. in "CHURCH FATHERS: Letter 22 (Jerome)", New Advent, translated by W.H. Fremantle, G. Lewis and W.G. Martley. From Nicene and Post-Nicene Fathers, Second Series, Vol. 6. Edited by Philip Schaff and Henry Wace. (Buffalo, NY: Christian Literature Publishing Co., 1893.) Revised and edited for New Advent by Kevin Knight