Source: Shaheed Murtada Mutahhari, The Narrative of Veracious, Vol.1, STORY, NO.61, p.138-139
During one of the journeys, the Holy Prophet (s) accompanied by his companions arrived at a barren and arid area. They were in need of fire—wood. The Prophet (s) said: “Gather some fire wood!”
The Companions replied: “0 Messenger of Allah (s)! Look! How barren this area is! One cannot find a single piece of wood here?
He (s) said: “Whatever it may be, everyone must gather a quantity of wood as much as he can.”
The Companions departed towards the desert. They were attentively looking at the ground to find the twigs. if they found a small piece they would pick it up. Everyone managed to gather as much wood as he could and brought it with him. When all of them pilled up what they had gathered, it became a great quantity of fire—wood.
Then the Holy Prophet (s) said: “Minor sins are just the same as these small pieces of wood. At the first sight, they seemed to be nothing, but everything has a searcher and a follower in the same way you have searched and researched till you gathered and piled up so much wood. Your sins are also amassed and recorded till the day that you will realize that these minor sins which were nothing at your sight, heaped up a plenty.”(1)

NOTES
1. A1-Wasa'il, v. 2, p. 462.