Once, in a holy place in the forest of Naimiṣāraṇya, great sages headed by the sage Śaunaka assembled to perform a great thousand-year sacrifice for the satisfaction of the Lord and His devotees.
Srimad Bhagavata Purana
श्रीमद्भागवतमहापुराणम्
मूल श्लोकः
नैमिषेऽनिमिषक्षेत्रे ऋषयः शौनकादयः । सत्रं स्वर्गाय लोकाय सहस्रसममासत
naimiṣe'nimiṣakṣetre ṛṣayaḥ śaunakādayaḥ | satraṃ svargāya lokāya sahasrasamamāsata
SB 1.1.4