Ambrose of Milan — as excerpted in the Catena Aurea on Matthew 5:4

“(ubi sup.) When yon have done thus much, attained both poverty and meekness, remember that you are a sinner, mourn your sins, as He proceeds, Blessed are they that mourn. And it is suitable that the third blessing should be of those that mourn for sin, for it is the Trinity that forgives sin.”