It is narrated from Sayyiduna Ali (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "The eye is the bond of the private parts (meaning, as long as a person is awake, he is aware if his ablution breaks, but while sleeping, he does not know), therefore, whoever sleeps should perform ablution after waking up."
Hadith Referenceمسند احمد / مسند الخلفاء الراشدين / 887
Hadith Gradingحکم دارالسلام:إسناده ضعيف لتدليس بقية بن الوليد الحمصي، والوضين بن عطاء مختلف فيه. وعبدالرحمن بن عائذ حديثه عن على مرسل