All Praise to Allah Subhanahu wa Taala and blessings of Him be on Prophet Muhammad peace be upon him His family and companions,
Say, [O Muhammad], "If you should love Allah , then follow me, [so] Allah will love you and forgive you your sins. And Allah is Forgiving
and Merciful."Quran 3:31
Acts or traditions of Prophet Muhammad peace be upon him are known as Sunnah.His acts or Sunan are of two types:
1.Sunan Zawaid: Those acts or conducts which he adhered to as His habit like in daily routine such as eating ,drinking dressing or part of tradition in arab are referred to as Sunan Al-Zawaid.
2.Sunan Huda: Those acts which Rasulullah peace be upon him adhered with in terms of part of worship which were not traditional but he did and became part of the Shariah are referred as Sunan Al-Huda.
There are Sunnahs of eating ,drinking talking ,sitting but "Breathing" is not an optional act to adopt ,it is natural which everyone is supposed to do without any intentions in order to live,so if you breath by making an intention that you would live a life to follow the teachings and acts of Rasulullah peace be upon him then In sha Allah you would be rewarded.
Allah swt and His Messenger knows best.