The Prophet ﷺ said, "When one of you wakes up from sleep at night, he should not put his hand into the vessel until he has poured water over it two or three times, for he does not know where his hand spent the night."
حدثنا أبو الوليد أحمد بن بكار الدمشقي يقال هو من ولد بسر بن أرطاة صاحب النبي ﷺ حدثنا الوليد بن مسلم عن الأوزاعي عن الزهري عن سعيد بن المسيب وأبي سلمة
عن النبي ﷺ قال إذا استيقظ أحدكم من الليل فلا يدخل يده في الإناء حتى يفرغ عليها مرتين أو ثلاثا فإنه لا يدري أين باتت يده
Sourcing, sahih. Muslim also narrated it, in the Book of Purification, chapter on the dislike of one who is performing wudu, or anyone else, dipping his hand while it is uncertain... etc, hadith 278, from Zuhri through this same route. There is also a report on this subject from Ibn Umar [Ibn Majah, hadith 394], Jabir [Ibn Majah, hadith 395], and Ayesha [Tayalisi in his Musnad, hadith 1487, though Abu Zura found a defect in it, Ilal al-Hadith 1/62, hadith 162].