Flight TK-65 with 134 passengers was on its way from Bangkok to Istanbul.
The sources said the emergency landing was made after crew saw a message on a toilet mirror which read 'bomb in cargo hold'.
The threat message was written using a lipstick.
The pilot then contacted Nagpur Air Traffic Controller who guided them to Delhi ATC.
According to the sources, all passengers and crew members will be interrogated by joint teams of intelligence and security agencies.
All baggage especially handbags are to be searched to match the lipstick tone and colour with the threat message.