It is narrated from Sayyiduna Anas (may Allah be pleased with him) that the Companions (may Allah be pleased with them) would fall asleep (while sitting), then stand up and pray. These people did not perform ablution (on account of it) in the time of the Prophet (peace and blessings be upon him) (either).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 591