Mujahid said:

'Umra' means that a man says to another man: It belongs to you so long as you live. When he says that, it belongs to him and to his heirs. Ruqba means that a man says to another: From me and from you.

حَدَّثَنَا عَبْدُ اللَّهِ بْنُ الْجَرَّاحِ، عَنْ عُبَيْدِ اللَّهِ بْنِ مُوسَى، عَنْ عُثْمَانَ بْنِ الأَسْوَدِ، عَنْ مُجَاهِدٍ، قَالَ الْعُمْرَى أَنْ يَقُولَ الرَّجُلُ لِلرَّجُلِ هُوَ لَكَ مَا عِشْتَ فَإِذَا قَالَ ذَلِكَ فَهُوَ لَهُ وَلِوَرَثَتِهِ وَالرُّقْبَى هُوَ أَنْ يَقُولَ الإِنْسَانُ هُوَ لِلآخِرِ مِنِّي وَمِنْكَ ‏.‏