I believed only actions of our Prophet were counted as Sunnah. But, I came across the ruling that turning our head while saying ‘Hayya alas-Salaah’ and ‘Hayya alal-Falaah’ during the Adhan to be a Sunnah. Although, the action is reported to be done by Bilaal(R.A) in Ahadith and reports of Prophet giving the Adhaan are rare and weak and has no such description either. So, what conditions made this qualify to be a Sunnah?
Although the actual action was performed by Bilal, but this was done in the presence of the Prophet and it is implied that it was with his instruction and approval. So it is proven from Sunnah, the action of the Prophet being to command or approve it.
Actions that the Sahaba did in the absence of the Prophet, for example after his death, will not be from Sunnah since we do not know whether the Prophet approved of them. These can then be either Ijtihad or Ijmah.