Anwar along with his supporters met Gandhi at his Tughlaq Lane residence and was welcomed into the party fold, the sources said.
The former MP from Katihar Lok Sabha constituency would make a formal announcement of his joining the Congress party later in the day, they said.
Anwar announced he was quitting the NCP and was also giving up his Lok Sabha membership on September 28 after Sharad Pawar's "defence" of Prime Minister Narendra Modi on the Rafale fighter jet deal. Pawar was quoted in the media as having given a clean chit to Modi on the deal.
Pawar, however, had clarified he was misquoted in the reports and he had given no such clean chit to Modi.
A former president of Congress's Bihar unit, Anwar formed the NCP in the 1990s along with Pawar and the late P A Sangma, opposing Sonia Gandhi taking over as AICC president on grounds of her foreign origin.
The party thereafter allied with the Congress at the national level and also in Maharashtra.