Chandogya Upanishad 1.10

When the Kurus had been destroyed by (hail) stones, Ushasti Kâkrâyana lived as a beggar with his virgin wife at Ibhyagrâma.

Seeing a chief eating beans, he begged of him. The chief said: 'I have no more, except those which are put away for me here.'

Ushasti said: 'Give me to eat of them.' He gave him the beans, and said: 'There is something to drink also.' Then said Ushasti: 'If I drank of it, I should have drunk what was left by another, and is therefore unclean.'

The chief said: 'Were not those beans also left over and therefore unclean?' 'No,' he replied; 'for I should not have lived, if I had not eaten them, but the drinking of water would be mere pleasure.'

Having eaten himself, Ushasti gave the remaining beans to his wife. But she, having eaten before, took them and put them away.

Rising the next morning, Ushasti said to her: 'Alas, if we could only get some food, we might gain a little wealth. The king here is going to offer a sacrifice, he should choose me for all the priestly offices.'

His wife said to him: 'Look, here are those beans of yours.' Having eaten them, he went to the sacrifice which was being performed.

He went and sat down on the orchestra near the Udgâtris, who were going to sing their hymns of praise. And he said to the Prastotri (the leader):

'Prastotri, if you, without knowing the deity which belongs to the prastâva (the hymns &c. of the Prastotri), are going to sing it, your head will fall off.'

In the same manner he addressed the Udgâtri; 'Udgâtri, if you, without knowing the deity which belongs to the udgtîha (the hymns of the Udgâtri), are going to sing it, your head will fall off.'

In the same manner he addressed the Pratihartri; 'Pratihartri, if you, without knowing the deity which belongs to the pratihâra (the hymns of the Pratihartri), are going to sing it, your head will fall off.' They stopped, and sat down in silence.