Abdullaah ibn Abbaas (RA) said, “do not sit with the people of innovation, for verily their sittings are a sickness for the hearts” [ash-Sharee'ah pg 65 of al-Aajurree (d.360)]
Mu’aadh bin Jabal (RA) used to say, whenever he sat in a circle of knowledge, “and I warn you of what is innovated, for all that is innovated is misguidance” [ash-Sharee'ah pg.55, also Abu Dawood with similar wording]
Abdullaah ibn Mas’ud (RA) said, “follow the sunnah of Muhammad and do not innovate, for what you have been commanded is enough for you.” [ad-Daarimee]
Hudhayfah bin al-Yaman said, “every act of worship that the Companions did not do, do not do it” [Abu Dawood]
Beware of made up duaas because while they have blessings, if you say them all the time instead of, say, the athkar that the prophet (S) said every day and night, you are missing out on far better blessings and forgetting the sunnah.