Musnad Ahmad 938:75

Ibrahim ibn Khalid narrated to us, Rabah narrated to us, from Umar ibn Habib, from Amr ibn Dinar, from Tawus, from Hajar al-Madari, from Zayd ibn Thabit, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Do not practice *ruqba* (a form of conditional gift). Whoever practices *ruqba*, the path is that of inheritance."