Umra is when a man says to another, "It is yours for as long as you live." When he says this, it belongs to him and to his heirs. Ruqba is when a person says, "It belongs to whichever of us, you or me, dies later."
حدثنا عبد الله بن الجراح عن عبيد الله بن موسى عن عثمان بن الأسود
ى أن يقول الرجل للرجل هو لك ما عشت فإذا قال ذلك فهو له ولورثته والرقبى هو أن يقول الإنسان هو للآخر مني ومنك
No references are available.
[Its isnad is hasan] Bayhaqi transmitted it: 6/176, from the hadith of Abu Dawud, through this route.