Tosefta zu Ejchah 3:41

The order [of the service] for fast days -- how [is it performed]? They take out the Ark to the open space of the city, and place burnt ashes on it. And they would not take turns (מתחלפין (see Lieberman)); rather, one person would sit and watch over it the whole day. The Elder who is with them says before them words of encouragement: "My sons, a person may be ashamed for his fellow, but a person should not be ashamed for his deeds. It is better that a person be ashamed for his fellow, and let him and his children not persist in hunger, and so it says (Is. 58:3), 'Why did we fast and You did not see? We afflicted our souls, and You did not know?' What can be inferred from this? 'Because on your fast day, [you pursue your business and exact all your payments]. Because you fast for strife and contentiousness, etc. Is such the fast that I desire, a day that a person afflicts his soul, etc.? Rather, this is the fast that I choose: To unfasten the bonds of wickedness, etc.' (Is. 58:3-6.) If there is a sheretz in a person's hand, even if he were to immerse it in the Shiloah, and in all the waters of Creation, he would not be purified. [But] if he casts a sheretz from his hand, he may immerse himself in [a mikveh of] forty seah of water, [and his immersion] is effective. And thus it says (Prov. 28:13), 'He who confesses and forsakes [his transgressions] will be granted mercy.' And it says (Lam. 3:41), 'Let us lift up our hearts with our palms, etc.'"