https://www.dhammatalks.org/suttas/DN/DN16.html
Then four leading Mallans, having bathed their heads and wearing new linen cloth, thinking, “We’ll ignite the Blessed One’s pyre,” were unable to ignite it. So the Kusinārā Mallans asked Ven. Anuruddha, “What is the reason, Ven. Anuruddha, what is the cause, why these four leading Mallans, having bathed their heads and wearing new linen cloth, thinking, ‘We’ll ignite the Blessed One’s pyre,’ are unable to ignite it?”
“Your intention, Vāsiṭṭhas, is one thing. The devas’ intention is another.”
“But what, Ven. Anuruddha, is the devas’ intention?”
“The devas’ intention, Vāsiṭṭhas, is, ‘This Ven. Mahā Kassapa is traveling on the highway from Pāvā to Kusinārā with a large Saṅgha of monks, approximately 500 monks in all. The Blessed One’s pyre will not catch fire until Ven. Mahā Kassapa has worshipped the Blessed One’s feet with his bathed head.’”
“Then let it be, venerable sir, in line with the devas’ intention.”
So Ven. Mahā Kassapa went to the Blessed One’s pyre at Makuṭa-bandhana, the Mallans’ memorial near Kusinārā. On arrival, arranging his robe over one shoulder, he placed his hands palm-to-palm over his heart, circumambulated the pyre three times, uncovered the Blessed One’s feet,56 and worshipped them with his head. And the 500 monks, arranging their robes over one shoulder, placed their hands palm-to-palm over their hearts, circumambulated the pyre three times, and worshipped the Blessed One’s feet with their heads. As soon as it had been worshipped by Ven. Mahā Kassapa and the 500 monks, the Blessed One’s pyre caught fire of its own accord. In the burning of the Blessed One’s body, no cinder or ash of the outer skin, inner skin, flesh, tendons, or oil of the joints could be discerned. Only the bone-relics57 remained. Just as in the burning of ghee or oil, no cinder or ash can be discerned; in the same way, in the burning of the Blessed One’s body, no cinder or ash of the outer skin, inner skin, flesh, tendons, or oil of the joints could be discerned. Only the bone-relics remained. And of the five hundred twin-wrappings, only two were burnt: the innermost & the outermost.