Natyashastra
वृत्तान्योजकृतानि स्युर्दुःखे यानि भवन्ति हि । तानि द्रुतासु योज्यानि लघुयुग्मकृतानि तु ॥ ४१॥
Metres which are irregularly constructed (ayojaka) and are associated with sorrowful situations are to be applied in the Drutā Dhruvās; and those constructed with pairs of short syllables (laghu-yugma) are likewise to be employed in them.
english translation
vRttAnyojakRtAni syurduHkhe yAni bhavanti hi । tAni drutAsu yojyAni laghuyugmakRtAni tu ॥ 41॥
hk transliteration by Sanscript