According to the CISM Review Manual, the incident response team should preserve the evidence as the first step to prepare for a third-party forensics investigation, as it helps to maintain the integrity and admissibility of the evidence in a court of law. Preserving the evidence may include isolating and imaging the infected systems, but these are not the only actions required. Cleaning the malware may destroy or alter the evidence and should be avoided until the investigation is completed.
Reference = CISM Review Manual, 27th Edition, Chapter 3, Section 3.6.2, page 165