Musnad Ahmad 1156:16

Jarir ibn Abd al-Hamid narrated to us, from Mansur, from al-Hakam, from Miqsam, from Umm Salamah, who said: "The Messenger of Allah (peace and blessings of Allah be upon him) used to perform Witr with seven [rak'ahs] and with five [rak'ahs], not separating between them with a taslim or with speech."