SN 54.13: To Ananda
Concentration through mindfulness of in-&-out breathing, when developed & pursued, brings the four establishings of mindfulness to completion. The four establishings of mindfulness, when developed & pursued, bring the seven factors for awakening to completion. The seven factors for awakening, when developed & pursued, bring clear knowing & release to completion.
For the longer (and more famous) sutta on mindfulness of breathing, see MN 118.