Anish Yadav (30), a gym trainer, had hatched plan to kill Ravi Kumar, who was married to the woman the former was in relationship with.
Yadav contacted Prem who allegedly agreed to kill Ravi by giving the poisonous injection. Yadav allegedly paid Prem Rs 1.5 lakh for it.
During interrogation, Prem said he had stolen the drug from a renowned government hospital a few days back, police said, adding they are verifying his claims and will contact the hospital concerned.
Prem was caught while he had administered the drug to Ravi, who succumbed during treatment.
Police said Yadav was also involved in an alleged case of gang-rape in 2010 in southwest Delhi's Dwarka area and is currently out on bail.