B.G 9.19
tapāmyahamahaṁ varṣaṁ nigr̥hṇāmyutsr̥jāmi ca। amr̥taṁ caiva mr̥tyuśca sadasaccāhamarjuna ॥19॥
I am the source of heat, and I withhold and send forth the rain. O Arjuna, know that I encompass both immortality and death, and I am simultaneously the cause and effect.
Gīta Tātparya 9.19
Due to the completeness of virtuous qualities, supreme Lord Vishnu is called 'sat'. As no one else is superior in qualities to Lord Vishnu, i.e. He is beyond ordinary qualities. Hence he is spoken of as 'asat'.
"sat sādhuguṇapūrṇatvādasmānnānyo guṇādhikaḥ। yato'to'saditiproktaṁ viṣṇvākhyaṁ paramaṁ padam॥"
"Due to the completeness of virtuous qualities, supreme Lord Vishnu is called 'sat'; no one else is superior in qualities to Lord Vishnu, i.e. He is beyond ordinary qualities, hence he is spoken as 'asat'."
iti śabdanirṇaye ॥16-19॥
- stated thus in the śabdanirṇaya, a book on the determination of words.