The 131st mitzvah is that we are commanded to make a proclamation of the kindness G‑d has bestowed upon us; and that we have separated the obligatory ma'asros and terumos. We must verbally clear ourselves from them just as we have physically cleared them from our premises. This is called vidui ma'aser.1

The source of this commandment is G‑d's statement,2 "And you must declare before G‑d your L‑rd: 'I have removed all the sacred portions from my house; I have given the appropriate ones to the Levite and to the orphan and widow....' "

The details of this mitzvah, and the manner and process of removing the tithes are explained in the last chapter of tractate Ma'aser Sheni.