Shama-il Tirmidhi Chapter 040, Hadith Number 002 (273).

Anas Radiyallahu 'Anhu reports: "Rasulullah Sallallahu 'Alayhi Wasallam performed six rak'ahs of the salaatut duha".

Commentary
Due to different times, different number of rak'ahs have been narrated regarding the salaatut duha of Sayyidina Rasulullah Sallallahu 'Alayhi Wasallam. Therefore there will be no contradiction between them. For this reason the 'ulama have written that at least two rak'ahs, and it is best that eight or twelve rak'ahs be performed. It has been narrated that Sayyidina Rasulullah Sallallahu 'Alahi Wasallam usually performed eight rak'ahs.