Abdullah bin Maslama reported,

Malik said that the statement of Umar bin Khattab , "What is separate is not to be combined, nor is what is combined to be separated," refers to the following situation. If each of several men owns forty sheep individually, and when the collector of zakat approaches, they pool their flocks together so that only a single sheep is due from the combined herd, this is forbidden.As for not separating what is combined, this means that when two partners each own a hundred and one sheep, three sheep are due from them collectively. But if, when the collector approaches, they divide their flocks, then only one sheep would be due from each of them. This is what I have heard concerning this matter.

حدثنا عبد الله بن مسلمة قال

قال مالك وقول عمر بن الخطاب رضي الله عنه لا يجمع بين متفرق ولا يفرق بين مجتمع هو أن يكون لكل رجل أربعون شاة فإذا أظلهم المصدق جمعوها لئلا يكون فيها إلا شاة ولا يفرق بين مجتمع أن الخليطين إذا كان لكل واحد منهما مائة شاة وشاة فيكون عليهما فيها ثلاث شياه فإذا أظلهما المصدق فرقا غنمهما فلم يكن على كل واحد منهما إلا شاة فهذا الذي سمعت في ذلك