Shatapatha Brahmana 9.1.1.20

9.1.1.20

teṣāṃ vā ubhayatonamaskārā anye | 'nyataratonamaskārā anye te ha te ghoratarā aśāntatarā ya ubhayatonamaskārā ubhayata evainānetadyajñena namaskāreṇa śamayati

Now some of these (formulas) have 'reverence' on both sides, and others on one side only;--more terrible and more unappeased, indeed, are those (Rudras) that have 'reverence' on both sides: on both sides he thereby appeases them by sacrifice, by reverence.