It is proven from the Prophet (peace and blessings of Allaah be upon him) that prayer in al-Masjid al-Haraam is better than a hundred thousand prayers (offered elsewhere). See Irwa’ al-Ghaleel 91129). 

The scholars differed as to whether this multiple reward applies only to the five prescribed prayers or does it include others as well? 

Al-Nawawi (may Allaah have mercy on him) said in Sharh Muslim: It should be noted that our view is that the superiority of prayer in these two mosques does not apply only to the obligatory prayers, rather it includes both obligatory and naafil (supererogatory) prayers. This is the view of Mutarrif among the companions of Maalik. Al-Tahhaawi said: It applies only to the obligatory prayers, but this is contrary to the general meaning of these saheeh ahaadeeth. And Allaah knows best. End quote. 

Shaykh Ibn ‘Uthaymeen (may Allaah have mercy on him) was asked: Does janaazah prayer in al-Haram al-Makki bring a multiple reward like other prayers? 

He replied: There is a difference of opinion among the scholars concerning this. Some scholars say that what brings the multiple reward in al-Masjid al-Haraam is the five daily prayers only, and other prayers do not bring the multiple reward. What appears to be the case from the apparent meaning of the hadeeth is that it includes all prayers, and the janaazah prayer is included in this general meaning of the reward for prayers in al-Masjid al-Haraam being multiplied. And Allaah knows best. End quote from Liqa’ al-baab il-Maftooh (116/11).

And Allaah knows best.