A Sunnah which we’ve abandoned is waiting for the next obligatory Salah after we have completed one.
Abu Huraira reported:
The Messenger of Allah (ﷺ) said: Should I not suggest to you that by which Allah obliterates the sins and elevates the ranks (of a man). They (the hearers) said: Yes, Messenger of Allah. He said: Performing the ablution thoroughly despite odds, tranverside of more paces towards the mosque, and waiting for the next prayer after observing a prayer, and that is mindfulness (Ar-Ribat).
(Saheeh Muslim, 251a)
Saheehus-Seerah 