Is the Hadith mentioned in "Musnad Ahmad," number (860), regarding the Companions' dancing before the Prophet Muhammad, peace be upon him, authentic? And is it permissible to dance and jump in such a manner to please Allah, especially since some Sufis use it as proof for their dancing?

The hadith (definition) mentioned in the question is weak due to two defects: the first is the unknown status of "Hani' ibn Hani'", and the second is the tadlis (deceptive narration) of "Abu Ishaq Al-Subai'i". Researchers of Musnad Imam Ahmad have classified it as weak. Furthermore, it has another chain of narration that is also weak and mursal (missing a Companion in the chain). Even if its authenticity were assumed, it contains no evidence for dancing in a dhikr circle. Rather, it is an expression of joy at the Prophet's (peace be upon him) praise, by leaping on one leg. This act is permissible and cannot be used as evidence for dancing during the remembrance of Allah. The jurists have explicitly stated that this act is an innovation (bid'ah), condemning it and considering it a sin and amusement, and among the most reprehensible acts of foolishness that only an ignorant person would commit.