It is narrated from Sayyiduna Ruwayfi' bin Thabit (may Allah be pleased with him): In the time of the Prophet (peace and blessings be upon him), one of us would take his brother's lean camel on the condition that half of whatever he gained as spoils would be for him and half for us; and one of us would get the arrowhead and the feathers, and the other the shaft. Then the Messenger of Allah (peace and blessings be upon him) said to me: "O Ruwayfi', perhaps your life will be prolonged after me, so inform the people that whoever ties knots in his beard, or hangs a bowstring around his neck, or cleans himself with an animal's dung or a bone, Muhammad (peace and blessings be upon him) is free of him."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 534