Before a pit sārvadhātuka affix beginning with a consonant, the augment iṭ is placed after brū,
As ब्रवीति, ब्रवीषि, ब्रवीमि, अब्रवीत् ॥ But ब्रवाणि before such an affix beginning with a vowel, and ब्रूतः before an affix which is not पित् ॥,
ब्रुव ईट्
Adhyāya 7 · Pāda 3 · Rule 93