In this way, my Lord, You appear in various incarnations as a human being, an animal, a great saint, a demigod, a fish or a tortoise, thus maintaining the entire creation in different planetary systems and killing the demoniac principles. According to the age, O my Lord, You protect the principles of religion. In the Age of Kali, however, You do not assert Yourself as the Supreme Personality of Godhead, and therefore You are known as Triyuga, or the Lord who appears in three yugas.
Srimad Bhagavata Purana
श्रीमद्भागवतमहापुराणम्
मूल श्लोकः
इत्थं नृतिर्यगृषिदेवझषावतारै- र्लोकान् विभावयसि हंसि जगत्प्रतीपान् । धर्मं महापुरुष पासि युगानुवृत्तं छन्नः कलौ यदभवस्त्रियुगोऽथ स त्वम्
itthaṃ nṛtiryagṛṣidevajhaṣāvatārai- rlokān vibhāvayasi haṃsi jagatpratīpān | dharmaṃ mahāpuruṣa pāsi yugānuvṛttaṃ channaḥ kalau yadabhavastriyugo'tha sa tvam
SB 7.9.38