Musnad Ahmad 27:353

Waki’ narrated to us, from Yazid ibn Ziyad, from Habib ibn Abi Thabit, from Ibn Umar, who said: The Messenger of Allah (peace and blessings of Allah be upon him) forbade *al-ruqba* (a form of conditional gift). And he said: "Whoever is given a *ruqba*, it is for him."