1. Immigration rules - Part8 - Family members - 281- i- (a)ii
2. Immigration Directorate Instructions - Chapter8 - Section FM1.21 - Partners & Parents English Language Requirement.
There are also numerous FOI requests where UKVI clearly state their required scoring is for Speaking and Listening only and that scores for Reading and Writing are disregarded.The applicant provides an original English language test certificate in speaking and listening from an English language test provider approved by the Secretary of State for these purposes, which clearly shows the applicant's name and the qualification obtained....
Hope that helps you.