Sayyiduna Abu Umamah (may Allah be pleased with him) said: "The Messenger of Allah (peace and blessings be upon him) used to pray nine rak'ahs of witr; then, when he (peace and blessings be upon him) felt it heavy upon him, he began praying seven rak'ahs of witr."
Hadith Referenceالمعجم الكبير للطبراني / باب الصاد / 8035