D.C. Code § 42-522: Estates at will; termination; creation.

An estate at will is one held by the joint will of lessor and lessee, and which may be terminated at any time, as herein elsewhere provided, by either party; and such estate shall not exist or be created except by express contract; provided, however, that in case of a sale of real estate under mortgage or deed of trust or execution, and a conveyance thereof to the purchaser, the grantor in such mortgage or deed of trust, execution defendant, or those in possession claiming under him, shall be held and construed to be tenants at will, except in the case of a tenant holding under an unexpired lease for years, in writing, antedating the mortgage or deed of trust.