This wonderful, unbelievable practice comes from Lord Buddha’s teachings found in the Kangyur, in the section on tantra.

When we make offerings of incense (in powder or stick form), flowers, food, etc., recite the following mantra 21 times and then offer them to the Three Jewels:

NAMAH SARVA TATHAGATHA AVALOKITE OM SAMBHARA SAMBHARA HUNG

The benefits of this mantra are as follows:

1) The food offered becomes divine (of the divine substance of the Buddha) with the blessing of this mantra;
2) It becomes an offering to the Buddhas, Dharma, and Sangha abiding in the ten directions. We accumulate an inconceivable amount of merit;
3) The merit created is equal to having praised all the Three Jewels;
4) The merit created is equal to having made a request to the Three Jewels;
5) We create the same amount of merit as from having rejoiced at the qualities of the Three Jewels;
6) At all times, the Buddhas are always admiring, paying attention to, and thinking of us, just how parents are always thinking of their sons and daughters;
7) Many devas are always protecting and guiding us;
8) The paramita of charity is completed.

The Buddha told Ananda, “Ananda, just as I have taught this method to you, reflect on it, practice it, keep it in mind, and reveal it to all sentient beings so that they will be able to see and hear this practice. In this way, one will achieve limitless merit.”

More Teachings by Rinpoche on this topic:
How to Make Offerings and Prayers in Front of Holy Objects