B.G 9.19
तपाम्यहमहं वर्षं निगृह्णाम्युत्सृजामि च। अमृतं चैव मृत्युश्च सदसच्चाहमर्जुन ॥१९॥
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'.
"सत् साधुगुणपूर्णत्वादस्मान्नान्यो गुणाधिकः। यतोऽतोऽसदितिप्रोक्तं विष्ण्वाख्यं परमं पदम्॥"
"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'."
इति शब्दनिर्णये ॥१६-१९॥
- stated thus in the śabdanirṇaya, a book on the determination of words.