It is narrated from Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) that the Prophet (peace and blessings be upon him) said: "The price of a dog is impure, and it (the dog itself) is even more impure than it." (The hadith scholars say that its chain contains) Yusuf bin Khalid al-Samti, and other narrators are more trustworthy and reliable than him.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 61