Pranavopanishat 8

8

makāraścāgnisaṃkāśo vidhūmo vidyutopamaḥ |
tisro mātrāstathā jñeyāḥ somasūryāgnitejasaḥ ||

and the 'm'-sound resembles fire, smokeless, like lightning. / Thus the three morae are to be known, possessing the brilliance of the moon, sun, and fire.