One verse

Srimad Bhagavatam 9.4.29

9.4.29

आरिराधयिषु: कृष्णं महिष्या तुल्यशीलया । युक्त: सांवत्सरं वीरो दधार द्वादशीव्रतम् ॥ २९ ॥

ariradhayisuh krsnam mahisya tulya-silaya yuktah samvatsaram viro dadhara dvadasi-vratam

ariradhayisuh
aspiring to worship
krsnam
the Supreme Lord, Krsna
mahisya
with his queen
tulya-silaya
who was equally as qualified as Maharaja Ambarisa
yuktah
together
samvatsaram
for one year
virah
the King
dadhara
accepted
dvadasi-vratam
the vow for observing Ekadasi and Dvadasi.

To worship Lord Krsna, Maharaja Ambarisa, along with his queen, who was equally qualified, observed the vow of Ekadasi and Dvadasi for one year.