Sajjan Paswan's body was found on Sunday morning at Sah's house, the police said.
A 33-year-old man was arrested on Tuesday for allegedly killing his wife's lover in northwest Delhi's Mahendra Park, the police said.
The accused, identified as Vinod Sah, told the police that on Saturday night his wife called Sajjan Paswan to meet her while he was sleeping. After he woke up, he found Mr Paswan and his wife together, the police said.
Enraged, he allegedly hit Mr Paswan on his head with a brick and fled with his family, they said.
Mr Paswan's body was found on Sunday morning at Sah's house, the police said.
Sah and his family were found missing and a manhunt was launched to find them. On Tuesday, police learnt about Sah and his family being present at the Old Delhi railway station following which Sah was arrested, the police said.
During interrogation, he told the police he was planning to flee to his village in Bihar with his family, said a senior police officer.