Rabbi Yosef Yitzchak of Lubavitch told:
Rabbi Schneur Zalman of Liadi returned from his first visit to Mezeritch–where he had first been exposed to the teachings of Chassidism as a disciple of Rabbi DovBer of Mezeritch–just prior to Passover of the year 5525 (1765). Though a young man of but twenty years at the time, Rabbi Schneur Zalman was already a respected figure in his hometown of Vitebsk, and many of the town’s young scholars regarded him as their teacher and mentor.
Upon his return to Vitebsk, Rabbi Schneur Zalman said to his disciples: “Soon we shall conduct the search for chametz, which Torah law instructs to be held ‘On the eve of the fourteenth, by the light of a candle’ and to cover all ‘recesses and crevices’of our homes. Searching for chametz involves more than removing every particle of physical leaven from our domain; it also means eradicating every last vestige of spiritual leaven–self-inflating pride–from every ‘recess and crevice’ of the fourteen elements of our personality: the seven character traits of one’s ‘animal self’ and the seven character traits of one’s ‘G-dly self.’ This, too, is a search to be conducted ‘by the light of a candle’–by the light of one’s ‘Candle of G-d, the soul of man.’”
On the thirteenth of Nissan of that year, Rabbi Schneur Zalman was so preoccupied with his preparations for the search for chametz that he ate nothing the entire day (he did not fast–it is forbidden to fast during the month of Nissan–he just didn’t eat). The search took all night, though Rabbi Schneur Zalman and his wife lived in a single room at the time.
From the Rebbe’s notes on the Passover Haggadah
 Of Nissan—i.e. the night prior to the seder night.
 Talmud, Pesachim 2a
 Shulchan Aruch, Orach Chaim, 431:1.
 See Tanya, chapters 3 and 6
 Proverbs 20:27.
 Shulchan Aruch, Orach Chaim, 430:2.
 Haggadah Shel Pesach Im Likkutei Ta’amim U’Minhagim, pg. 2.