Mbh 6.120.1h

And all around in the welkin he heard celestial voices saying, ‘Why, Oh why, should Ganga’s son, that foremost of all warriors of weapons, yield up his life during the southern declension?’ Hearing these words, the son of Ganga answered, ‘I am alive!’ Although fallen upon the earth, the Kuru grandsire Bhishma, expectant of the northern declension, suffered not his life to depart. Ascertaining that to be his resolve, Ganga, the daughter of Himavat, sent unto him the great Rishis in swanlike form. Then those Rishis in the forms of swans inhabiting the Manasa lake, quickly rose up, and came together, for obtaining a sight of the Kuru grandsire Bhishma, to that spot where that foremost of men was lying on his bed of arrows. Then those Rishis in swanlike forms, coming to Bhishma, beheld that perpetuator of Kuru’s race lying on his bed of arrows. Beholding that high-souled son of Ganga, that chief of the Bharatas, they walked round him, and the Sun being then in the southern solstice, they said, addressing one another, these words, ‘Being a high-souled person, why should Bhishma pass out (of the world) during the southern declension?’ Having said these words, those swans went away, proceeding towards the southern direction. Endued with great intelligence, Bhishma, O Bharata. beholding them, reflected for a moment. And the son of Santanu then said unto them, ‘I will never pass out (of the world) as long as the Sun is in the southern solstice. Even this is my resolve. I will proceed to my own ancient abode when the Sun reacheth the northern solstice. Ye swans, I tell you this truly. Expectant of the northern declension I will hold my life. Since I have the fullest control over the yielding up of my life, I will, therefore, hold life, expectant of death during the northern declension. The boon that was granted to me by my illustrious sire, to the effect that my death would depend on my own wish. O, let that boon become true.