Ata' bin Abi Rabah (may Allah have mercy on him) said: I prayed the Isha prayer beside Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both). When he finished, he said: "Shall I not teach you the witr?" I said: "Of course." So he prayed one rak'ah.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4792