Mu'awiyah bin Abdullah bin Ja'far (may Allah have mercy on him) said that the Messenger of Allah (peace and blessings be upon him) said: "The pledge is not to be locked away." A man pledged a house in Madinah for a fixed term, and when the time of payment came, the one who had taken the pledge said: Now this house is mine. The Messenger of Allah (peace and blessings be upon him) said: "The pledge is not to be withheld." This is mursal.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الرهن / 11238