Midrasch zu Jechezkiel 44:20

40) (Vayikra 10:6): "Your heads al tifrau (lit., 'do not uncover')": I might take this to mean that you should not remove your hat, but we derive (otherwise), viz.: It is written here periah ("tifrau"), and elsewhere, (in respect to a Nazirite, Bamidbar 6:5), "periah." Just as "periah" there refers to letting the hair grow long; here, too, it refers to letting the hair grow long, (so that the translation above becomes "Your hair do not grow long" (in mourning).