Fitness for work certificate
You do not require a fitness for work certificate for any illness lasting seven days or less. Your employer may require you to complete a self-certification form (SC2) when you return to work which is available from your employer or on the HMRC website.
If you are unwell for more than seven days your employer can ask you to provide them with medical evidence to support payment of SSP (statutory sick pay). Proof of your illness typically comes from a fitness for work certificate which you request from your GP.
For more information see the DirectGov website.