Musannaf Ibn Abi Shayba 1:608

Abu Bakr narrated to us, he said: Hafs bin Ghiyath narrated to us, from al-A'mash, from Yahya bin Wathab, he said: Ibn Abbas was asked about a man who went out to the prayer and stepped on excrement. He said: "If it was moist, he should wash what touched him, and if it was dry, it does not harm him."