In their hatred of Joseph, his brothers arranged to sell him into slavery. To add to their sin, they deceived their father, Jacob:
And they took Joseph's coat, and killed a kid of the goats, and dipped the coat in the blood; And they sent the coat of many colours, and they brought it to their father; and said, This have we found: know now whether it be thy son's coat or no. And he knew it, and said, It is my son's coat; an evil beast hath devoured him; Joseph is without doubt rent in pieces. And Jacob rent his clothes, and put sackcloth upon his loins, and mourned for his son many days. And all his sons and all his daughters rose up to comfort him; but he refused to be comforted; and he said, For I will go down into the grave unto my son mourning. Thus his father wept for him. (Genesis 37:31-35)
To help you understand the significance of Jacob's response, The Dake Annotated Reference Bible offers the following explanation, on page 28 of the Old Testament:
Rending clothes is a chief sign of mourning in the East. It is customary to lay aside jewels and fine clothing and to be clothed in sackcloth while making loud wailing. See Josh. 7:6; 1 Sam. 4:12; 2 Sam. 1:11; 3:31; 13:31; 2 Ki. 2:12; 18:37; 19:1; Ezra 9:3; Job 1:20. The ceremony of rending clothes requires a knife. Cuts are first made on the right side of the upper garment for brother, sister, son, daughter, or wife, and on the left side for a father or mother. Other garments are cut likewise. Sackcloth is generally made of the hair of goats or camels, and is very coarse and black. The same material was used for straining liquids, and making sacks. Sackcloth could be worn either over or under the outer garments, or instead of them.