The dust of the feet of a devotee, the water that has washed the feet of a devotee, and the remnants of food left by a devotee are three very powerful substances. By rendering service to these three, one attains the supreme goal of ecstatic love for Krsna. In all the revealed scriptures this is loudly declared again and again. Therefore, my dear devotees, please hear from me, for I insist again and again: please keep faith in these three and render service to them without hesitation. From these three one achieves the highest goal of life – ecstatic love of Krsna. This is the greatest mercy of Lord Krsna.

Srila Prabhupada, Cc Antya 16.60-63, translation