Simak bin Harb says that he heard Sayyiduna Jabir bin Samurah (may Allah be pleased with him) say: "The white hairs of the Prophet (peace and blessings be upon him), when he (peace and blessings be upon him) combed his beard (I think he said: and oiled it) would not be visible, and when he (peace and blessings be upon him) left it as it was, they would become visible."
Hadith Referenceالمعجم الكبير للطبراني / باب الجيم / 1877