A sunnat can be established through a da῾īf hadīth. However to establish a sunnat mu’akkadah (emphasised sunnat) one will need a qawī (strong) narration.
Hadhrat then said, “I had a copy of Sifr-us-Sa῾ādah. I recorded in its footnotes all those ahādīth through which sunnats are established.
Malfoozat Mufti Mehmood Hasan Gangohi rahmatullah alayh, Pg. 125