חסידות אויף בראשית 4:5

"One may only search for chametz by the light of a candle" (Pesachim 2a). Ner (נר, "candle") contains the letters of neshamah (נ) and ruach (ר) [i.e., the initials of two levels of the soul]. And specifically with a wax candle — sha'avah (שעוה, "wax") contains the letters shin-ayin (שע) and vav-heh (וה). Shin-ayin (שע) is from the language of the verse "but to Cain and his offering He did not turn (sha'ah)" (Genesis 4:5) — meaning that one must unify and turn toward the Name Y-H (י"ה) with V-H (ו"ה) in every mitzvah a person performs, just as we say before every mitzvah: le-shem yichud Kudsha Berikh Hu u-Shekhinteih — "to unify the Name Y-H with V-H." But the verbal recitation is not the essential thing, for "it is the intention of the heart that is the essence of the matter" — the main thing is that one directs one's heart toward Heaven. And for this reason, specifically a wax candle is required — to hint at all of the above.