It is narrated from Sayyiduna Amr bin Kharijah (may Allah be pleased with him): I was holding the reins of the she-camel of the Messenger of Allah (peace and blessings be upon him) while it was chewing the cud, and its saliva was flowing onto my shoulders. Then he narrated the long hadith.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 1206