One of the errors of Imam Ahmad b. Hanbal was his belief that the spirit of the deceased returns to its body in the grave:
أَنَّهُ يَقْبِضُ الأَرْوَاحُ ثُمَّ تُرَدُّ في الأَجْسَادِ في القُبُورِ
(And we believe that) the Angel of Death takes the souls, then returns them to the bodies in the graves.
Source: Tabaqatul Hanabilah, vol. 2, p. 430