Question: Which UFC weight division did Chris Leben and Anderson Silva compete in?

Context paragraphs:
Andrea Derjaj: Andrea Derjaj (born 26 February 1996) is an Albanian boxer who competes in the 91 kg weight division.  He is a 3-time defending national champion in his weight division and is a member of Albania national youth team.

Kalib Starnes: Kalib Axel Starnes (pronounced ['kaɪlʌb 'æksʌl 'stɑhrnz]; born January 6, 1975) is a Canadian mixed martial artist.  He was a semi-finalist on SpikeTV's "The Ultimate Fighter", training under UFC Hall of Famer Ken Shamrock.  He holds notable victories over Chris Leben and Jason MacDonald.  Starnes is currently under contract with World Series of Fighting, fighting in their light heavyweight division.  He is currently ranked as the 2nd light heavyweight mixed martial artist in Canada.  Kalib is a native of Surrey, British Columbia and trains with Aegis Athletics in Surrey, BC.

Chris Leben: Christian Cyrus Leben (born July 21, 1980) is a retired American mixed martial artist.  He formerly competed with the UFC as a Middleweight, compiling a record of 12-10 in 22 appearances for the organization.  Leben first appeared in the inaugural season of "The Ultimate Fighter" reality series, and has also fought in the now-defunct WEC.  He began his UFC career with five victories in a row before suffering a knockout loss to future UFC Middleweight Champion Anderson Silva.  Leben was the inaugural WEC Middleweight Champion.

Anderson Silva: Anderson da Silva (] ; born April 14, 1975) is a Brazilian mixed martial artist and former UFC Middleweight Champion.  Silva holds the longest title streak in UFC history, which ended in 2013 after 2,457 days, with 16 consecutive wins and 10 title defenses.  He has 13 post-fight bonuses, the second most in UFC history.  UFC president Dana White and several mixed-martial-arts publications have called Silva the greatest mixed martial artist of all time.  He is currently ranked the #6 contender in official UFC middleweight rankings.

Romulo Barral: Rômulo Barral (born May 3, 1983) is a Brazilian Jiu Jitsu competitor.  He is a black belt under Vinicius Magalhaes and competes for Gracie Barra, where he has won numerous championships.  Rômulo Barral is one of the top Brazilian Jiu Jitsu fighters in the “Meio Pesado” weight division.  Rômulo Barral maintained this black belt status in the years that followed with consistent medals at the top BJJ tournaments in the world.  In August, 2011, Rômulo established his own Gracie Barra academy in Northridge, California, proving that he is also a coach.  Rômulo is a 5-time black belt world champion, 3-time silver medalist in the open weight division, and a NO GI world champion, and the 2013 ADCC champion.

UFC 126: Former Light Heavyweight Champion Vitor Belfort was scheduled to fight Yushin Okami at UFC 122.  However, UFC President Dana White said that Belfort would instead challenge Middleweight Champion Anderson Silva for the belt.  Belfort was originally scheduled to compete against Silva on January 2, 2010 at UFC 108, but due to an injury sustained by Silva, the fight had to be postponed until February 6, 2010 at UFC 109 and April 10, 2010 at UFC 112, respectively.  However, both fights were cancelled.

UFC 138: UFC 138: Leben vs. Muñoz was a mixed martial arts event held by the Ultimate Fighting Championship on November 5, 2011 at LG Arena in Birmingham, United Kingdom.  The event aired on the same day, via tape delay, on Spike TV in the United States and live on Sportsnet in Canada.  The event was the second that the UFC has hosted in Birmingham, the first was UFC 89 back in 2008 also featuring Chris Leben in the main event.

Edwin Dewees: Edwin Stanton Dewees (born August 7, 1982 in Florence, South Carolina) is an American mixed martial artist.  He was featured on "The Ultimate Fighter 4".  His official UFC record is 0–3, losing matches to Rich Franklin at UFC 44 by TKO, Chris Leben at UFC Ultimate Fight Night 2 by armbar, and Jorge Rivera at The Ultimate Fighter 4 finale by TKO.

Stephan Bonnar: Stephan Patrick Bonnar (born April 4, 1977) is an American professional wrestler and retired professional mixed martial artist and a UFC Hall of Famer who competed as a Light Heavyweight for the UFC.  Bonnar was the runner-up on The Ultimate Fighter 1.  His TUF Ultimate Finale loss to Forrest Griffin is considered to be one of the most important fights in the history of the UFC.  He is currently under contract for Bellator MMA.  For most of his MMA career Bonnar played the "role of the underdog", this is particularly exemplified in his fight with Anderson Silva at UFC 153.

Chris Weidman: Christopher James "Chris" Weidman (born June 17, 1984) is an American mixed martial artist.  He is the former UFC Middleweight Champion.  He is currently ranked as the #6 middleweight in the world.  In 2013, he earned worldwide prominence by becoming the first man to defeat Anderson Silva inside the UFC to become the Middleweight Champion, which he held for approximately two and a half years.  Chris Weidman successfully defended the belt three times before losing it to Luke Rockhold.

Supporting paragraphs: Chris Leben Anderson Silva
Answer: Middleweight