Musnad al-Humaydi

Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) narrates: The Noble Prophet (peace and blessings of Allah be upon him) forbade the transaction of "Habl al-Hablah" (i.e., making a deal for such a period that when a she-camel gives birth to its offspring, and then the she-camel born from that offspring gives birth to its own offspring, then making a deal for that offspring, or fixing the payment of another transaction at such an indeterminate time).