Thank you for the background on this Dhammakaya movement, I hadn’t encountered that yet.
But back to our original question:
I’ve finally found the story, although I am not exactly sure about the number 1250 (would have to check the previous part of the sutta and count all the arahants).
The story occurs in DN 14 from segment 3.24.1 onward:
DN14:3.24.1 ff:
Then in the late afternoon, the Buddha Vipassī came out of retreat and addressed the mendicants, telling them all that had happened. Then he said,
‘Wander forth, mendicants, for the welfare and happiness of the people, out of compassion for the world, for the benefit, welfare, and happiness of gods and humans. Let not two go by one road. Teach the Dhamma that’s good in the beginning, good in the middle, and good in the end, meaningful and well-phrased. And reveal a spiritual practice that’s entirely full and pure. There are beings with little dust in their eyes. They’re in decline because they haven’t heard the teaching. There will be those who understand the teaching!
But when six years have passed, you must all come to Bandhumatī to recite the monastic code.’
Then most of the mendicants departed to wander the country that very day. Now at that time there were 84,000 monasteries in India.
And when the first year came to an end the deities raised the cry:
‘Good sirs, the first year has ended. Now five years remain. When five years have passed, you must all go to Bandhumatī to recite the monastic code.’
And when the second year … the third year … the fourth year … the fifth year came to an end, the deities raised the cry:
‘Good sirs, the fifth year has ended. Now one year remains. When one year has passed, you must all go to Bandhumatī to recite the monastic code.’
And when the sixth year came to an end the deities raised the cry:
‘Good sirs, the sixth year has ended. Now is the time that you must go to Bandhumatī to recite the monastic code.’
Then that very day the mendicants went to Bandhumatī to recite the monastic code. Some went by their own psychic power, and some by the psychic power of the deities.
And there the Blessed One Vipassī, the perfected one, the fully awakened Buddha, recited the monastic code thus:
‘Patient acceptance is the ultimate fervor.
Extinguishment is the ultimate, say the Buddhas.
No true renunciate injures another,
nor does an ascetic hurt another.
Not to do any evil;
to embrace the good;
to purify one’s mind:
this is the instruction of the Buddhas.
Not speaking ill nor doing harm;
restraint in the monastic code;
moderation in eating;
staying in remote lodgings;
commitment to the higher mind—
this is the instruction of the Buddhas.’
Obviously the story has later been ascribed to “our” Buddha, as happened with other stories too. And it also differs in the detail that, in the version that I heard at least, there was no prior appointment, but here in DN 14 there was a prior appointment. And, although they had neither emails nor WhatsApp, they still had their very efficient heavenly messaging system!
Oh yes, and there’s even this parallel at SN4.5, but not with all the details, and especially without the ovada patimokkha part.