2.3.65 कर्तृकर्मणोः कृति

कृत्-suffix ending words combining have षष्ठी in कर्त्ता or कर्म. For example, कालिदासस्य कृतिः. Note that it is only in case of कृत् that the rule is applied - भुक्तपूर्वी उदनं doesn't have षष्ठी.