To worship Lord Kṛṣṇa, Mahārāja Ambarīṣa, along with his queen, who was equally qualified, observed the vow of Ekādaśī and Dvādaśī for one year.
Srimad Bhagavata Purana
श्रीमद्भागवतमहापुराणम्
मूल श्लोकः
आरिराधयिषुः कृष्णं महिष्या तुल्यशीलया । युक्तः सांवत्सरं वीरो दधार द्वादशीव्रतम्
ārirādhayiṣuḥ kṛṣṇaṃ mahiṣyā tulyaśīlayā | yuktaḥ sāṃvatsaraṃ vīro dadhāra dvādaśīvratam
SB 9.4.29