O Ruwayfi‘, you may live a long life after me; so, inform people that anyone who ties his beard, wears a string around his neck, or cleanse himself with animal dung or with bone, indeed Muhammad has nothing to do with him

The Prophet (may Allah's peace and blessings be upon him) informed this Companion that he would live a long life until he would see people contrary to his guidance. Such people would mess up with their beards by imitating the non-Arabs or the extravagant and effeminate people, instead of letting the beard grow and taking care of it.
Or they would violate the creed of monotheism by resorting to polytheistic methods like wearing necklaces or letting their animals wear it, believing that such things could ward off evil.
Or they would commit what their Prophet declared prohibited, i.e. using animal dung or bones for Istijmār.
Hence, the Prophet (may Allah's peace and blessings be upon him) asked his Companion to convey to his Ummah the fact that he disassociates himself from those who adopt any of these practices.