Sayyiduna Ibn Abbas (may Allah be pleased with them both) said: "Whoever washes a deceased person should take a ritual bath (ghusl)."
This is how it was narrated from him with this chain, but the authentic report from Ibn Abbas is contrary to this.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 1458