I've seen pictures that depict the Buddha floating in the air and emitting fire from this torso and water from his legs. The fire and the water might be the other way around. I think the picture illustrates a sutra or other text. Does anyone know what the original text might be?
It's called the "twin miracle": The DPPN entry under Yamaka pātihāriya says,
The Twin Miracle is described at DA.i.57, and in very great detail at DhA.iii.204; see also J.iv.263ff. etc.
After the Buddha returned to his father's kingdom, uncertainty still existed about whether Gautama Buddha was really enlightened or not. In response, the Buddha allegedly displayed the Yamaka-pātihāriya or the "Twin Miracle", called so because of its simultaneous production of apparently contradictory phenomena; in this case, fire and water.
The twin miracle entailed Gautama Buddha producing flames from the upper part of his body and streams of water from the lower part of his body, alternating this, and doing similarly between the left and right sides of his body.
Afterwards, the Buddha took three giant steps, arriving in Tavatimsa. There, he preached the Abhidharma to his mother who had been reborn there as a Deva named Santussita.
The fact that the Buddha could perform these miracles should be taken in the light to strengthen your devotion to the Buddha. If this is for pure delight or intellectual stimulations in miracles like watching or reading about magic or watching a magic show, this is the wrong motivation to investigate about it.
Having said the there is a Wikipedia page on miracles the Buddha performed including the twin miracle.
I am not exactly answering your question here but I hope this adds value.
The earliest source seems to be Paṭisambhidāmagga. See here https://discourse.suttacentral.net/t/sutta-searching-water-fire-miracle-in-pali-canon/5088/2
It appears to be elaborating a mere reference to something called “Twin Miracle” in Cūḷaniddesa, which in itself is a commentary on parts of the Suttanipata.