Reports instruct the washing of a vessel after a dog laps from it. Wulūgh means a dog lapping from a vessel.
إِذَا وَلَغَ الْكَلْبُ فِي إِنَاءِ أَحَدِكُمْ
Translation: When a dog laps from the vessel of one of you.
Hanafi reasoning takes the washing command as evidence of contamination from saliva. The dog's leftover water is consequently not used for ordinary wudu.