Tirumazhisai Azhwar is one of the twelve azhwars (or alwar), a group of Vaishnava saints who sang devotional poems on Lord Vishnu. Those poems are collectively known as Naalayira Divya Prabhandham.
Tirumazhisai Alwar was the son of Bhrigu but raised by a childless tribal couple. He proclaimed that he doesn't belong to any of the four varnas (Avarna) i.e., Brahmin, Vaishya, Kshatriya and Shudra. But there was a couple of incidents other people saw him by the caste he was raised in.
As a part of his journey, Alwar was travelling to Thirukudanthai Sarangapani temple, Kumbhakonam which is also known as Aravamudhar Sannidhi. Alwar stopped at a verndah and sat down. A group of brahmins were reciting Vedas. Because they saw a person who is not a dwija, they stopped reciting (There is a smriti injunction that Vedas should not be chanted in front of a non dwija). Alwar noticed it and moved very distant from them. Later, the brahmins wanted to resume the chanting where they stopped. But they couldn't remember it and clueless. Alwar realised their cluelessness and turned back. He took a black grain, broke it with his nail and dropped it down. By this action, they took the hint and continued the chanting.
kṛṣṇānāṃ vrīhīnām nakhanirbhinnaṃ nakhanirbhinnaṃ
कृष्णानां व्रीहीणां नखनिर्भिन्नं नखनिर्भिन्नं
At this point, those brahmins understood the alwar knew the Vedas but he doesn't open his mouth. They asked for his pardon.
What is the meaning and context of the above verse Alwar hinted to the Brahmins? Where is this mantra located?