Natyashastra
गायत्र्! याश्चापि शक्वर्याश्चान्तरे वा व्यवस्थिता । तेन पादप्रमाणेन ध्रुवा कार्याऽवसानिकी ॥ २०॥
And whether the Gāyatrī or the Śakvarī (metres) are arranged in between, the Dhruvā intended for the conclusion should be composed according to the measure of their pādas.
english translation
gAyatr! yAzcApi zakvaryAzcAntare vA vyavasthitA । tena pAdapramANena dhruvA kAryA'vasAnikI ॥ 20॥
hk transliteration by Sanscript