Dua for seeking FORGIVENESS

Rabbigh firli watub alayya innaka antat tawwabur raheem

O my Lord ! forgive me and accept my repentance , indeed You are Oft-Returning with Compassion and You are Merciful.

REFERENCE :

SUNAN E ARBA`

A companion of the Holy Prophet (P.B.U.H) said that we used to recite these words hundred times in an assembly.
( Ibn e Haban , Narrated by Ibn e Umer (R.A))