It is narrated from Sayyiduna Anas (may Allah be pleased with him) that the Companions (may Allah be pleased with them) would wait for the Isha prayer until their heads nodded from sleep; then they would pray and not perform ablution.
(b) Imam Abu Dawud (may Allah have mercy on him) says that Shu'bah, from Qatadah, added the words that this was the practice in the time of the Messenger of Allah (peace and blessings be upon him).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 590