Adbhutasagara 48.89

48.89

mārkaṇḍeyapurāṇe tvasyaiva svaptāhariṣṭatvamuktam|
yaścātihanyate bhūto dṛṭvā rātrāvatho divā|
sa mṛtyuṃ saptarātreṇa naraḥ prāpnotyasaṃśaye||

In the Mārkaṇḍeyapurāṇa, however, this very thing is stated to be a seven-day sign of death:
"A man who, having seen a spirit by night or by day, is beaten severely,
reaches death within seven nights, without doubt."