Source records available on Ancestry are OK—but as the previous answer said, if those same records are available on FamlySearch, where anyone can access them without paying fee, that is preferred. In both cases, but especially when citing Ancestry (or any other paid site) you should use a complete citation and not just a link, so that the reader of the profile can see where the information originated.
In contrast to source records available on Ancestry, citations to Ancestry family trees should be avoided, since the huge majority cite no sources, and most of the rest cite problematic sources like the AGBI, or cite sources that don't really support the assertion, such as citing a census record for an exact date and place of birth or for the names of an adult census entry's parents. If an Ancestry tree does cite a good, solid source, cite the source here, not the Ancestry tree.
In the rare situations where an Ancestry tree does have a lot of good sources, you might want to put the tree under Sources with a "see also" moniker—but for the basic facts of a person's life, birth, marriage, death, you should still cite the original sources.