Battle of Chi Ceti
From Halopedia, the Halo wiki
En route to the Damascus Testing Facility, the UNSC Commonwealth was attacked by a Covenant craft. The frigate successfully fended off the Covenant ship, blasting a hole in it with a MAC and then hitting it with a Shiva warhead. The ship retreated to the edge of the system.
The Spartans on board then landed on Chi Ceti 4, where they donned their MJOLNIR armor for the first time.
At this point, the Covenant ship turned around and continued it's attack on the system. The Master Chief decided that he and his Spartans were going to sneak onboard the ship, and blow it up from the inside. The Spartans boarded a Pelican, and took off into space. They strapped the Pelican's ANVIL-II ASM warheads to themselves and donned thruster packs to fly through space to get onboard the Covenant ship. Only three Spartans, the Master Chief, Kelly and Sam succesfully landed on the ship and entered through it's hole. They reached the ship's reactor, tossed their warheads inside, and escaped. Unfortunatly, Sam had been wounded and was forced to be left behind. He held off the Covenant while The Master Cheif and Kelly escaped. The warheads went off, and the Covenant ship was destroyed, taking Sam with it.