Question:
If a Companion (Ṣaḥābī) refers to an action as Sunnah, what is meant by it?
Source: Fatāwā Amunpuri by Shaykh Ghulam Mustafa Zaheer Amunpuri
Answer:
If a Companion describes an action as Sunnah, the meaning is that it is a marfūʿ ḥadīth (directly attributed to the Prophet ﷺ). The scholars are unanimous on this.
Scholarly Statements:
❀ Imām al-Ḥākim رحمه الله:
قد أجمعوا على أن قول الصحابي: سنة، حديث مسند
“There is consensus that when a Companion says ‘Sunnah’, it refers to a musnad ḥadīth (a narration attributed to the Prophet ﷺ).”
(al-Mustadrak, 1/358; Maʿrifat ʿUlūm al-Ḥadīth, p. 23)
❀ Ḥāfiẓ al-Bayhaqī رحمه الله:
إني لا أعلم خلافا بين أهل النقل أن الصحابي إذا قال: من السنة كذا وكذا أنه يريد به سنة رسول الله صلى الله عليه وسلم، ويكون ذلك مسندا
“To my knowledge, there is no disagreement among the scholars of transmission that when a Companion says ‘From the Sunnah is such-and-such’, he means the Sunnah of the Messenger of Allah ﷺ, and it is regarded as marfūʿ.”
(al-Khilāfiyyāt, 2/110, 2/441, 4/238)
❀ Ḥāfiẓ Ibn ʿAbd al-Barr رحمه الله (463 AH):
لا يختلفون فى ذلك
“The scholars do not differ on this matter.”
(al-Taqaṣṣī, 1/161)

When a Companion of the Prophet ﷺ calls something Sunnah, it is understood as a Prophetic Sunnah and therefore treated as a marfūʿ narration.