Sa'id bin al-Musayyib (may Allah have mercy on him) said: Taking a ritual bath after washing the deceased is Sunnah, and being in a state of ablution while lowering him into the grave until he is buried is also Sunnah. Apart from that, ablution is not required of anyone: neither of the one who offers his funeral prayer, nor of the one who carries his bier, nor of the one who walks with it. The statement of Ibn al-Musayyib (may Allah have mercy on him) has already passed: "If I knew that it was impure (najas), I would never touch it" (that is, in his view the deceased is not impure).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 1450