Fiqh-us-Sunnah Volume 1, Purification and Prayer, Fiqh 1.030.
Section : Washing the hands three times at the beginning.
This is based on the hadith of Aus ibn Aus al-Thaqafi who said, “I saw the Messenger of Allah make ablution, and he washed his hands three times.” (Related by Ahmad and an-Nasa’i.) Abu Hurairah reported that the Prophet, upon whom be peace, said, “When one of you rises from his sleep, he should not put his hand into a pot until he has washed it three times, for he does not know where his hand was (while he slept).” (Related by “the group”, al-Bukhari did not mention the number of times.)