Musar על חולין 182:8

In this way was all use they made of the things of this world. Since they were clinging to G-d's holiness, blessed be He, behold, it was an elevation and an enhancement for that thing which merited to be of use to a Tzadik (righteous person). Our sages already referred to the matter of the "stones at the place" which Yaakov took and put under his head: "Rabbi Yitzchak said: This tells us that all the stones gathered themselves together into one place and each one said: 'Upon me shall the righteous man rest his head'" (Chulin 91b).