Simak bin Harb reported,

I heard Jabir bin Samura being asked about the gray hair of the Prophet ﷺ. He said, "When he oiled his head, none of it could be seen, but when he did not oil it, it was visible."

عن سماك بن حرب يقول

سمعت جابر بن سمرة سئل عن شيب النبي ﷺ فقال: كان إذا دهن رأسه لم ير منه شيء، وإذا لم يدهن رئي منه.