Found it. For anyone in the future who's interested. Source: THE TREATISE ON THE GREAT VIRTUE OF WISDOM OF NĀGĀRJUNA (MAHĀPRAJÑĀPĀRAMITĀŚĀSTRA)
One day, accompanied by Ānanda, the Buddha went to Śrāvastī on his alms-round. A poor old woman was standing at the roadside. Ānanda said to the Buddha: "This woman is worthy of compassion; the Buddha should save her." The Buddha replied: "This woman does not have the conditions required [to be saved]."
Ānanda continued: "May the Buddha approach her. When she sees the Buddha with his major marks (lakṣaṇa) and minor marks (anuvyañjana) and his rays (raśmi), she will experience a joyful mind (muditācitta) and will thus fulfill the required conditions." Then the Buddha came near the woman, but she turned away and showed her back to him. The Buddha tried to approach her from four different sides; each time she turned her back to him in the same way. She looked up in the air, but when the Buddha came down to her, she lowered her head at once. The Buddha rose up from the earth [to make her see him], but she lowered her face with her hands and did not want to look at the Buddha. Then the Buddha said to Ānanda: "What more can I do? Everything is useless; there are people who do not fulfill the conditions necessary for being saved and who do not succeed in seeing the Buddha." That is why the Buddha has said that it is as difficult to meet a Buddha as a flower on the udumbara tree. With the Buddha, it is like rainwater (varṣajala), easy to receive in folded hands, but which the pretas, ever thirsty, never get.
Now that I know the source, an alternative translation was easy to find:
Source: Marvelous Stories From The Perfection Of Wisdom - Nagarjuna, translated by Bhikshu Dharmamitra (Kalavinka Press)
The Buddha Attempts to Connect with an Old Woman
Introductory Exegesis Discussion: The Rarity of Meeting a Buddha
The Buddha said that there is one situation which is difficult to encounter. It is that of meeting up with a buddha, one of the World Honored Ones. He also said that in a period of ninety-one kalpas, only three kalpas have buddhas whereas the rest of the kalpas are all empty. They have no buddha. It is extremely lamentable. The Buddha explained this circumstance for the sake of those who are burdened by severe karmic offenses and who have not planted the roots of goodness whereby they might see a buddha, stating, “The difficulty of encountering an age in which there is a buddha is like that of encountering the blossoms of the udumbara tree which appear only once in a great long time.” People with karmic offenses such as these turn about in the three wretched destinies. Perhaps they may come to abide among men or gods at a time when a buddha appears in the world but, still, these people do not succeed in seeing him even then. As it has been told, there were nine hundred thousand households in the city of Śrāvastī among whom three hundred thousand actually saw the Buddha, three hundred thousand heard of the existence of the Buddha but never saw him, and the remaining three hundred thousand neither heard of nor saw him. Although the Buddha dwelt in the state of Śrāvastī for twenty-five years, still, these beings neither heard of him nor saw him. How much the less did those who dwelt at a distance.
Story: The Buddha Attempts to Connect with an Old Woman
In addition, we have the exemplary case wherein the Buddha once went together with Ānanda into the city of Śrāvastī seeking almsfood. At just that time there was a poverty-stricken old mother who was standing at the end of the street. Ānanda said to the Buddha, “This person is pitiable. The Buddha should bring her to deliverance.” The Buddha replied, “This person has no conducive causes and conditions.” Ānanda said, “Would that the Buddha would just go forth and approach her. When this person sees the Buddha’s special characteristics and becomes aware of the light which he radiates, she will develop a delighted mind and will thereby create conducive causes and conditions.”
The Buddha then went forth and approached her with the result that she turned her body so that her back was towards the Buddha. The Buddha then approached her from each of the four sides and in each case she turned her back towards the Buddha and raised her head so as to gaze upwards. And so the Buddha approached from above with the result that she then lowered her head so as to look downwards. The Buddha emerged from the ground beneath her, but she then covered her eyes with both hands and, even then, was unable to gaze at the Buddha. The Buddha then inquired of Ānanda, “Well, just what further causes and conditions would you wish to have created now?”
Nāgārjuna’s Concluding Comments
There are people such as these who are lacking in the causes and conditions requisite to being brought to deliverance. They are unable to succeed in seeing the Buddha. It is for this reason that the Buddha said that the rarity of being able to encounter the Buddha is comparable to that of being able to see the blossoms of the udumbara tree. This is analogous to the situation of the hungry ghosts who, even when rainwater is abundant and easily found everywhere, still remain constantly thirsty and unable to find anything at all to drink.