I was recently informed that occasionally the Prophet (saw), whilst travelling, prayed the full Salah rather than the two Rak’aat. Is this true and will the Salah be valid?

It is not permissible for a traveller to perform the full prayer i.e. 4 Rakats. Almighty Allāh has instructed the traveller to perform Qasr and this is obligatory without choice. There are certain traditions that suggest that the Prophet (saw) performed the full 4 Rak`aat occasionally. (See Dārqutnī 1:242, Bayhaqī 3:142) However, all these traditions are related by unreliable narrators. (Zaadul-Ma`ād, 1:464) If someone however performs 4 Rak`aat, then according to the Hanafi school of thought, his Salāh will only be valid if he performs Sajdatus-Sahw in which case the extra 2 Rak`aat will be regarded as Nafl. (Raddul-Muhtār)