Sahih Muslim Book 4, Hadith Number 1236.

Chapter : Desirability of seeking refuge from the torment of the grave.

Abu Zubair reported: Abdullah b Zubair used to say La ilaha il-Allah at the end of every prayer like the Hadith narrated by Ibn Numair and he reported it in the end, and then reported Ibn Zubair saying: The Messenger of Allah (may peace be upon him) uttered La ilaha il-Allah at the end of every prayer.