Halakhah к Шмуэль Б 13:31

When a person suffers the loss [the death] of one for whom he is obligated to mourn, he must tear his garments because of it. He must tear his garments while standing, as it is said, "And the king arose and tore his garments."¹II Samuel 13:31. If he tore his garments while sitting, he has not fulfilled his obligation, and he must again tear his garments while standing. Initially he should tear them before the deceased is covered in the grave, when his sorrow is still intense.