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The effective spin Hamiltonian is constructed in the framework of the almosthalf-filled Hubbard model on the Cayley tree by means of functional integraltechnique with the use of static approximation. The system in the ground stateappears to be consisting of the ferromagnetic metallic domains and theantiferromagnetic insulating one sprovided that the concentration of excesselectrons (or holes) does not exceed some critical value. The connectionbetween the Hubbard model and the {\it s-f} model is stated. Note added: Thisposting is intended to show that the phase separation in strongly correlatedelectronic systems was considered already in 1970-80's and is not a discoveryof latest manganites boom

The Effective Spin Hamiltonian and Phase Separation Instability of the Almost Half-Filled Hubbard Model and Narrow-Band {\it S-f} Model