There are many narrations in which Muhammad is alleged to have said that the tailbone of a human being will not decay, and be the bone from which God will recreate people when the resurrection takes place:
Sahih al-Bukhari 4814
Narrated Abu Huraira:
The Prophet (ﷺ) said, "Between the two blowing of the trumpet there will be forty." The people said, "O Abu Huraira! Forty days?" I refused to reply. They said, "Forty years?" I refused to reply and added: Everything of the human body will decay except the coccyx bone (of the tail) and from that bone Allah will reconstruct the whole body.
This is also narrated in Sahih al-Bukhari 4935, Sahih Muslim 2955a-c, and Sunan an-Nasai 2077.
As an aside, the tailbone decays just like every other bone. All bones on a human being decay, and can be destroyed. Muhammad's alleged statement is therefore scientifically inaccurate.
The teaching that the tailbone is incorruptible, and that God will use it to recreate people, appears to come from Jewish Midrash:
Genesis Rabbah 28
3 "The Lord said: I will obliterate man [haadam]" - Rabbi Levi said in the name of Rabbi Yoḥanan: Even the lower millstone was obliterated. Rabbi Yehuda bar Simon said in the name of Rabbi Yoḥanan: Even the dust of Adam the first man was obliterated. When Rabbi Yehuda expounded this in public in Tzippori, they did not accept it from him. Rabbi Yoḥanan said in the name of Rabbi Shimon ben Yehotzadak said: Even the sacrum, from which the Holy One blessed be He will cause man to sprout in the future, was obliterated.
Hadrian, may his bones be crushed, once asked Rabbi Yehoshua ben Ḥananya, saying to him: From where will the Holy One blessed be He cause man to sprout in the future? He said to him: From the sacrum. He said to him: From where do you know this? He said to him: Bring one to me and I will show you. He ground it in a mill, but it did not become ground up. He placed it in water but it did not dissolve. He placed in on an anvil and began striking it with a hammer; the anvil split and the hammer broke and it [the sacrum] lacked nothing.
This is also taught in Kohelet Rabbah 12:5.
Therefore, not only is the statement regarding the tailbone in the Hadith false, as all bones decay, it is also borrowed from non-inspired Jewish commentary from the early centuries AD.
Muhammad repeated many Jewish legends and teachings in the Quran, and apparently, was also narrating such doctrines to his companions.