The Cattle - 6:19
"Ask: "Whose testimony is the greatest?"Say: "Allah is the witness between me and you. I have been given this Qur´an to warn you and all those to whom it reaches. Do you bear witness that there is another god besides Allah?"Say: "I do not bear witness to that,"and then say: "He is the only God, and I am far from that which you associate partners with."
Related:
The Women/4:166
But Allah bears witness to what He has sent down to you. He sent it down with His knowledge. The angels also bear witness to it. Allah is sufficient as a witness.