It is narrated from Sayyidah Umm Salamah (may Allah be pleased with her) that the Messenger of Allah (peace and blessings be upon him) used to pray witr with thirteen rak'ahs; then, when he (peace and blessings be upon him) grew older and weaker, he began to pray witr with seven rak'ahs.
Hadith Referenceالمعجم الكبير للطبراني / مسند النساء / 20045