After the third takbeer in janaazah salaah, one should recite the following dua:
اللّهُمَّ أَنْتَ رَبُّهَا وَأَنْتَ خَلَقْتَهَا، وَأَنْتَ هَدَيتَهَا إِلَى الإسْلَامِ، وَأَنْتَ قَبَضْتَ رُوحَهَا، وَأَنْتَ أَعْلَمُ بِسِرِّهَا وَعَلَانِيَتِهَا، جِئْنَا شُفَعَاءَ فَاغْفِرْ لَهُ
O Allah, You are the Rabb of this janaazah (i.e. this deceased person), You have created him, You guided him to Islam, and You took away his soul. You are most knowledgeable regarding his secret life and his apparent life. We have come seeking intercession (on his behalf). Therefore, forgive him.
وعن أبي هريرة عن النبي صلى الله عليه وسلم في الصلاة على الجنازة: اللهم أنت ربها وأنت خلقتها، وأنت هديتها إلى الإسلام، وأنت قبضت روحها، وأنت أعلم بسرها وعلانيتها، جئنا شفعاء فاغفر له (سنن أبي داود، الرقم: ٣٢٠٠، مشكاة المصابيح، الرقم: 1688)
Hazrat Abu Hurairah (radhiyallahu ‘anhu) reports that Rasulullah (sallallahu ‘alaihi wasallam) would recite the abovementioned dua in the janaazah salaah.