Gently bend your neck, Sigamber; worship what you burned; burn what you worshipped.
Mitis depone colla, Sigamber; adora quod incendisti, incende quod adorasti.
Original text (Latin)
Source, in full
Advice to King Clovis (25 December 496) in Gregory of Tours, Historia Francorum, bk. 2, sec. 31 (tr. Ernest Brehaut, 1916). French quoted in Norbert Guterman, A Book of French Quotations (1963), p. vi: "Adore ce que tu as brûlé, brûle ce que tu as adoré."