AU-C Section 200.19 (Part 1 of 3): Sufficiency and Appropriateness of Audit Evidence

AU-C Section 200.19 says:

"To obtain reasonable assurance, the auditor should obtain sufficient appropriate audit evidence to reduce audit risk to an acceptably low level and thereby enable the auditor to draw reasonable conclusions on which to base the auditor's opinion." 

Audit evidence is required to support the auditor's opinion, and is comprised of information (or a lack of information) that supports or contradicts management's assertions; this evidence can come from several places:

• procedures performed over the course of the audit;
• previous audits;
• a firm's quality control procedures;
• the entity's accounting records; or
• information provided by a specialist employed by the entity.

The auditor should seek to obtain evidence that is sufficient and appropriate to support his opinion.  Sufficiency (i.e., quantity) and appropriateness (i.e., quality) of evidence are inversely related.  For example, if the quality of the evidence is high, the auditor might determine he doesn't need as much evidence to support the assertion.  Whether or not sufficient appropriate audit evidence has been obtained is a matter of professional judgment and explained more in detail at AU-C Section 500.

https://www.aicpa.org/content/dam/aicpa/research/standards/auditattest/downloadabledocuments/au-c-00200.pdf