Normally we would advise you not to open any messages purporting to be from HMRC in case they are phishing, however in this case if you have claimed for tax credits you may receive confirmation by text message.
Extract from www.hmrc.gov.uk
HMRC will be sending text messages (SMS messages) to some customers between 20 May and 5 August 2014. The text messages will acknowledge receipt of claims for tax credits, and tax credit renewals made by post.
The SMS message will not ask you to provide any personal or financial information.