Secret Sadaka

It is narrated from Amr ibn Thabit that when Zaynul Abidin (grandson of the fourth righteous caliph Ali ibn Abu Talib) died , people washed him and saw black marks on his back.

And they began to ask: “What are these black footprints from?”

They were told that Zaynul Abidin had loaded sacks of flour on himself at night and distributed it to the poor in Medina.

See Khilyatu al-Awliya vol. 3, p. 136.