Is the prohibition of dipping one's hand into a vessel before washing it three times upon waking from night sleep for obligation or for recommendation, and what is its evidence?

The jurists differed on the ruling of washing hands before inserting them into a vessel upon waking up from sleep. The majority of scholars held it to be recommendable (mustahabb), while the Hanbalis considered it obligatory, especially upon waking from night sleep, based on the agreed-upon hadith of Abu Hurayrah: "When one of you wakes up from his sleep, he should not put his hand into the vessel until he has washed it three times, for he does not know where his hand has spent the night." The majority of scholars view that the hadith is to be understood as recommendable due to its reasoning, "for he does not know where his hand has spent the night," and because doubt does not affect the certainty of purity, and due to the generality of the verse from Surah Al-Ma'idah which does not mention washing the hands at the beginning of wudu'.