The Majjhima Nikāya quoted in The Buddha Speaks, a Book of Guidance from the Buddhist Scriptures, Edited by Anne Bancroft (2010):

Find a place where you are alone. Train yourself in the following way: When you breathe in, experience breathing in. When you breathe out, be fully conscious that you are breathing out. If you cherish and practice this, it will bear great fruit. Whatever you are doing and wherever you are, you will find steadiness, calm and concentration if you become conscious of your breathing.