Śāṅkhāyana (Kauṣītaki) Āraṇyaka 2.6

2.6

ŚĀ 2,6
athaitāni caturuttarāṇi śaṃsati tad anūkam | tā ekaviṃśatir ṛco bhavanti | ekaviṃśatir vā asyānūkasya parvāṇi bhavanti | tānyevaitābhiḥ saṃdadhāti | tāni vai sapta tṛcāni bhavanti | sapta vai chandāṃsi sarveṣām eva chandasām āptyai | tānyarkavanti bhavanti | tad etasyāhno rūpam ||

ŚĀ 2.6
Then he recites these [verses] increasing by four: that is the backbone. They are twenty-one verses. Twenty-one indeed are the joints of this backbone. He unites them with these [verses]. They are seven triplets. Seven indeed are the metres, for the attainment of all the metres. They contain the word arka. That is the form of this day.