Tosefta do Sofoniasza 1:19

On the evening of the fourteenth, we check for chamets by candlelight, not by sunlight, not by moonlight, but rather only by candlelight (since the candlelight is greater). Even though there's no explicit proof for this, it's a remembrance for that which is discussed (Tzefaniah 1) "And in that time I will look through Jerusalem by candle" and that which it says (Proverbs 20) "The candle of God is a human soul".