Consider the following excerpt from the book Mahabharata,
"Vaisampayana continued, 'Hearing these words of Drupada, the virtuous king Yudhishthira replied, saying, 'O great king, I also shall have to marry.' Hearing him, Drupada said, 'If it pleaseth thee, take thou the hand of my daughter thyself with due rites. Or, give Krishna in marriage unto whomsoever of thy brothers thou likest.' Yudhishthira said, 'Thy daughter, O king, shall be the common wife of us all! Even thus it hath been ordered, O monarch, by our mother. I am unmarried still, and Bhima also is so amongst the sons of Pandu. This thy jewel of a daughter hath been won by Arjuna. This, O king, is the rule with us; to ever enjoy equally a jewel that we may obtain. O best of monarchs, that rule of conduct we cannot now abandon. Krishna, therefore, shall become the wedded wife of us all.
Since Yudhishthira never lie, there are 2 reasons for all Pandavas to marry Draupadi. The reasons are
Order from mother (Kunti told all Pandavas to marry Draupadi)
All Pandavas equally enjoy a jewel won by anyone
Now my doubt is that where can (in which paragraph of Mahabharata) I find the explicit statements from Kunti saying those words from 1)?