Abu Ayyub Ansaari Radiyallahu'Anhu says. "Rasulullah Sallallahu 'Alayhi Wasallam always performed four rak'ahs around the time of zawaal. I said:
'O Messenger of Allah. You give much importance to these four rak'ahs around the time of zawaal of the sun?' Rasulullah Sallallahu 'Alayhi Wasallam replied: 'The doors of the skies open from the zawaal of the sun till the zuhr salaah is performed. I desire that a good deed of mine reaches the skies at that time'. I asked: 'Is there a recital in every rak'ah?' He replied: 'Yes'. I enquired: 'Should salaam be made at the end of two rak'ahs?' Rasulullah Sallallahu 'Alayhi Wasallam replied 'No' (Salaam should be made only at the end of four rak'ahs)".
English reference: Book 40, Hadith 277
Arabic reference: Book 41, Hadith 293