The freeing of slaves.
Translation endnotes
[90:13] Appreciative people, monotheists, cannot have slaves. To be a slave owner is equivalent to claiming to be a "lord" and thus tantamount to polytheism. See 4:3,25,92; 2:286; 3:79; 5:89; 8:67; 24:32-33; 58:3-4; 90:13; 2:286; 12:39-42; 79:24.