Classical novae are quite frequent in M 31. However, very few spectra of M 31 novae have been studied to date, especially during the early decline phase. Our aim is to study the photometric and spectral evolution of a M 31 nova event close to outburst. Here, we present photometric and spectroscopic observations of M31N 2005-09c, a classical nova in the disk of M 31, using the 1.3 m telescope of the Skinakas Observatory in Crete (Greece), starting on the 28th September, i.e. about 5 days after outburst, and ending on the 5th October 2005, i.e. about 12 days after outburst. We also have supplementary photometric observations from the La Sagra Observatory in Northern Andalucía, Spain, on September 29 and 30, October 3, 6 and 9 and November 1, 2005. The wavelength range covered by the spectra is from 3565 Å to 8365 Å. The spectra are of high S/N allowing the study of the evolution of the equivalent widths of the Balmer lines, as well as the identification of non-Balmer lines.The nova displays a typical early decline spectrum that is characterized by many weak Fe II multiplet emissions. It is classified as a Pfe nova. From the nova light curve, we have also derived its speed class, t_2=14±2.5 days. As the nova evolved the Balmer lines became stronger and narrower. The early decline of the expansion velocity of the nova follows a power law in time with an exponent of ≃-0.2.