"At the time of Buddha, there lived an old beggar woman called Relying on Joy. She used to watch the kings, princes, and people making offerings to Buddha and his disciples, and there was nothing she would have liked more than to be able to do the same. But she could only beg enough oil to fill a single lamp. However, as she placed it before Buddha she made this wish: "I have nothing to offer but this tiny lamp. But through this offering, in the future may I be blessed with the lamp of wisdom. May I free all beings from their darkness. May I purify all their obscurations, and lead them to enlightenment."
That night, the oil in all the other lamps went out. But the beggar woman's lamp was still burning at dawn, when Buddha's great disciple Maudgalyayana came to collect the lamps. He saw no reason why one lamp was still alight and tried to snuff it out. But whatever he did, the lamp kept burning.
Buddha had been watching all along, and said: "Maudgalyayana, do you want to put out that lamp? You cannot. You could not even move it, let alone put it out. If you were to pour the water from all the oceans over this lamp, it still wouldn't go out. The water in all the rivers and lakes of the world could not extinguish it. Why not? Because this lamp was offered with devotion, and with purity of heart and mind. And that motivation has made it of tremendous benefit." "
retold by Sogyal Rinpoche