Your spouse must complete, sign and provide you with Form 8332. This is used when the custodial parent (one whom the child spend more than 6 months during the year) releases the right to let the other parent ( noncustodial parent) to claim the child.
In that case, the noncustodial parent will be entitled to the exemption/dependency and the child tax credi. The custodial parent is granted the Earned Income Credit.