Consider the following excerpt from The Mahabharata:
SECTION CLXLIII (Swayamvara Parva 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.
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 lies, there are 2 reasons for all Pandavas to marry Draupadi:
Order from their mother (i.e., Kunti telling all five Pandavas to marry Draupadi)
All Pandavas equally enjoy a jewel won by anyone (source already cited above)
Now my doubt is, where (i.e., in which paragraph of Mahabharata) can I find the explicit statements made by Kunti to support reason #1?