Musnad Ahmad 30:2741

Muhammad ibn Ja‘far narrated to us, he said: Shu‘bah narrated to us, from Khalid, from ‘Abdullah ibn Shaqiq, from Abu Hurayrah, from the Prophet, may Allah bless him and grant him peace, that he said: "When one of you wakes up from his sleep, let him not dip his hand into his vessel until he has washed it three times, for he does not know where his hand spent the night."