Natyashastra
मध्यस्थादिषु भावेषु मुखरागः प्रयोक्तृभिः । प्रसन्नस्त्वद्भुते कार्यो हास्यशृङ्गारयोस्तथा ॥ १६४॥
For neutral and similar states, the facial complexion should be natural. In the wondrous (adbhuta) sentiment, it should be bright, and in the comic (hāsya) and erotic (śṛṅgāra) sentiments, it should also be pleasant.
english translation
madhyasthAdiSu bhAveSu mukharAgaH prayoktRbhiH । prasannastvadbhute kAryo hAsyazaRGgArayostathA ॥ 164॥
hk transliteration by Sanscript