I know that Bhishma narrates this story to Duryodhana. Please give me the reference from the Mahabharata if you can. It would help.
This story in mentioned in Mahabharata, Udyoga Parva, SECTION XLIX.
"Bhishma continued, 'Having heard these words, Sakra went to the spot where those two were practising ascetic austerities, accompanied by all the celestials and having Vrihaspati at their head. At that time, the dwellers of heaven had been very much alarmed in consequence of a war raging between themselves and the Asuras. And Indra asked that illustrious couple to grant him a boon. Thus solicited, O best of the Bharata race, those two said,--Name thou the boon.--Upon this Sakra said unto them,--Give us your aid.--They then said unto Sakra,--We will do what thou wishest. And then it was with their aid that Sakra subsequently vanquished the Daityas and the Danavas.