´It was narrated from Ibn 'Abbas that:` The Prophet got up during the night and went to the toilet and relieved himself, then he washed his face and hands, and went back to sleep. (Sahih) Another chain with similar wording.
Hadith Referenceسنن ابن ماجه / كتاب الطهارة وسننها / 508
Hadith Gradingالألبانی:صحيح | زبیر علی زئی:بخاري ومسلم
Commentary: (1) Sleeping in a state of ablution (wudu) is a source of reward. (Sahih al-Bukhari, Book of Ablution, Chapter: The Virtue of One Who Sleeps in a State of Ablution, Hadith: 247 and Sahih Muslim, Book of Remembrance and Supplication, Chapter: What to Say When Going to Sleep and Lying Down, Hadith: 2710) However, sleeping in a state of ablution is not obligatory. Washing the hands and face is also sufficient; in fact, there is no harm in sleeping without ablution, even if there is a need for a ritual bath (ghusl). As will be mentioned in Hadiths: 581 to 583.
Source: Commentary on Sunan Ibn Mājah by Mawlānā ‘Atā’ullāh Sājid, Page: 508
Ibn Abbas said: The Messenger of Allah ﷺ got up at night, fulfilled his need and washed his face and hand and then slept. Abu Dawud said: that is to say, he urinated
Shaykh Umar Farooq Saeedi
Benefits and Issues: To sleep in a state of ablution (wudu) means that not only should one perform ablution and go to bed in the early part of the night, but also, if one wakes up during any part of the night and goes to relieve oneself or for any other need, then it is highly virtuous to perform the Sunnah ablution again before sleeping. This is a most excellent deed.
Source: Sunan Abu Dawood – Commentary by Shaykh Umar Farooq Saeedi, Page: 5043