x-amazon-apigateway-integration get lambda arn for uri property - amazon-web-services

Question

The x-amazon-apigateway-intregration extension has a URI property that takes an arn value of the backend. When I try to deploy this with SAM I get the following error
Unable to parse API definition because of a malformed integration at path /auth/signIn
How can I get the ARN value of my lambda function that is specified in my template?
Template:
"api" : {
"Type" : "AWS::Serverless::Api",
"Properties" : {
"StageName" : "prod",
"DefinitionUri" : "src/api/swagger.json"
}
},
"signIn": {
"Type": "AWS::Serverless::Function",
"Properties": {
"Handler": "index.signIn",
"Runtime": "nodejs8.10",
"CodeUri": "./src",
"FunctionName": "signIn",
"ReservedConcurrentExecutions": 15,
"Timeout": 5,
"Events": {
"SignIn": {
"Type": "Api",
"Properties": {
"Path": "/signIn",
"Method": "post",
"RestApiId" : {
"Ref": "api"
}
}
}
}
}
},
Swagger Definition:
"paths" : {
"/auth/signIn" : {
"post" : {
"x-amazon-apigateway-integration" : {
"httpMethod" : "POST",
"type" : "aws_proxy",
"uri" : {
"Fn::Sub" : {
"Fn::GetAtt": [
"signIn",
"Arn"
]}
}
},
"parameters" : [
{
"name" : "email",
"in" : "body",
"required" : "true",
"schema" : {
"type" : "string"
}
},
{
"name" : "password",
"in" : "body",
"required" : "true",
"schema" : {
"type" : "string"
}
}
],
"responses" : {
"200" : {
"description" : "Authenticated"
},
"default" : {
"description" : "Unexpected Error"
}
}
}
},