From Sahih Muslim (Hadith 587):
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
Reference: Muslim 587