Abu Mujliz says that I once asked Sayyiduna Ibn Abbas (may Allah be pleased with them both) about Witr, so he replied that I heard the Prophet (peace and blessings be upon him) say: "Witr is one rak'ah in the last part of the night." Then I asked Sayyiduna Ibn Umar (may Allah be pleased with them both) about this, and he also gave the same answer.
Hadith Referenceمسند احمد / ومن مسند بني هاشم / 3408