Maitreya, one should understand that any discourse that satisfies four conditions is what the Buddha spoke. What are the four? Maitreya, they are (1) when the discourse is meaningful instead of meaningless, (2) when it contains the Dharma as opposed to not containing the Dharma, (3) when it makes defilements decrease and does not make defilements increase, and (4) when it teaches the qualities and benefits of nirvāṇa and does not increase the defects of samsara. Maitreya, one should know that any discourse is what the Buddha spoke when it possesses these four factors.
Maitreya, no matter whether a discourse endowed with these four factors is offered, or to be offered, by a monk, a nun, or a male or female lay precept-holder, faithful sons or daughters of noble lineage should perceive that person to be the Buddha; perceiving them as the Teacher, they should listen to the sacred Dharma.
If you wonder why that is so, Maitreya, it is because everything that is well spoken —no matter what it is— is what the Buddha taught.
—Buddha Shakyamuni, Sutra Inspiring Determination
Great Middle Way 