Extending Buddha's lifespan Where did Lord Buddha say to Ananda Thero that a Buddha can, if there is an invitation, extend His lifespan? Was this mentioned in a Sutta or somewhere else?
It is DN 16, here

The Buddha said to him: “Ānanda, Vesālī is lovely. And the Udena, Gotamaka, Sattamba, Bahuputta, Sārandada, and Cāpāla shrines are all lovely.
Whoever has developed and cultivated the four bases of psychic power—made them a vehicle and a basis, kept them up, consolidated them, and properly implemented them—may, if they wish, live on for the eon or what’s left of the eon. The Realized One has developed and cultivated the four bases of psychic power, made them a vehicle and a basis, kept them up, consolidated them, and properly implemented them. If he wished, the Realized One could live on for the eon or what’s left of the eon.”
But Ānanda didn’t get it, even though the Buddha dropped such an obvious hint, such a clear sign. He didn’t beg the Buddha: “Sir, may the Blessed One please remain for the eon! May the Holy One please remain for the eon! That would be for the welfare and happiness of the people, for the benefit, welfare, and happiness of gods and humans.” For his mind was as if possessed by Māra.

... and here

Therefore, Ānanda, the misdeed is yours alone, the mistake is yours alone. For even though the Realized One dropped such an obvious hint, such a clear sign, you didn’t beg me to remain for the eon, or what’s left of it. If you had begged me, I would have refused you twice, but consented on the third time. Therefore, Ānanda, the misdeed is yours alone, the mistake is yours alone.