Sayyiduna Abdullah bin al-Zubayr (may Allah be pleased with them both) said: "When the Prophet (peace and blessings be upon him) prayed the Isha prayer, he would pray four rak'ahs and perform the witr with one prostration (rak'ah), then sleep, until he performed the prayer that came after his (first) prayer in some part of the night."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 13847