ויקרא כג:ח מדרש: ספרא

5) (Vayikra 23:8) "And you shall present a fire-offering to the L–rd for seven days." Whence is it derived that if you have no bullocks, bring rams; if you have no rams, bring sheep; if you have no clean ones, bring unclean ones? From "And you shall present a fire-offering to the L–rd" — in any event.