Midrash for Rosh Hashanah 50:15

א"ר זירא כגון שראוהו בלילה:

- It is necessary. For you might suppose that the testing of the witnesses is regarded as the commencement of a suit in court, and the pronouncement of 'sanctified', 'sanctified' as the end of the suit, and therefore they should sanctify at night, on the analogy of money suits, as we have learnt, 'Money suits are heard by day and concluded [if necessary] at night'; so here we should sanctify at night.

Sifrei Devarim