Several of the companions of Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) narrate that he used to say: "When one of you has prayed witr, then sleeps and gets up (at night), let him undo his witr by adding one more rak'ah to it, and then pray witr again after that."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 9395