assalamu alikum It is forbidden to build mosque on the graves but there is a hadith in al-Bukhari which state that Hadith 470: Translated by: Ustadha Aisha Bewley Abdullahrelated that the Prophet, may Allah bless him and grant him peace, prayed at the end of the upper part of the valley behind al-'Arj if you are on the way to Hadba.There are two or three graves at that mosque, on which are piles of stones, to the right of the path at the large stones marking the path. 'Abdullah used to return home from al-'Arj after the sun had declined from midday. He would pray Dhuhr in that mosque." so Abdullah bin Umar prayed in the mosque where there were 2 or 3 graves?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and messenger.

The Hadeeth to which you referred in the question does not indicate that the Prophet, sallallaahu ‘alayhi wa sallam, or ‘Abdullaah ibn ‘Umar offered prayer in the very spot where a grave or graves were found. The Hadeeth was reported by Al-Bukhari with the following wording, “… there were two or three graves by ('Inda in Arabic) that mosque, on those graves there were piles of stones. They were located to the right of the path…”

The Arabic word (‘inda), translated as “by” does not necessarily means “inside” or “next to”, so there is nothing to support the claim that the Prophet, sallallaahu ‘alayhi wa sallam, performed prayer in a place where there was a grave or next to a grave. It (the word ‘inda) can also mean “beside,” according to the Arabic dictionary, Al-Qaamoos Al-Muheet.

However, there are many authentic Ahaadeeth that are explicitly indicative of the forbiddance of taking graves as mosques. For example:

The Prophet, sallallaahu ‘alayhi wa sallam, said: “Beware of taking graves as mosques; I forbid you to do that.” [Muslim]

He, sallallaahu ‘alayhi wa sallam, also said: “Know that the most wicked people are those who take graves as places of prayer.” [Ahmad]

These explicit texts and others which indicate the prohibition of taking graves as places of (performing) prayer should not be left and replaced by such implicit texts (i.e. this Hadeeth). Verily, those who do so are like those who resort to ambiguous texts and overlook definitive and lucid ones.

Allaah Knows best.