Muwatta Imam Malik Book of Judgements, Hadith 13

Yahya said, "Malik related to us from Ibn Shihab from Sa'id ibn al-Musayyab that the Messenger of Allah ﷺ said, 'The pledge given as security is not forfeited.' "
Malik said, "The explanation of that according to what we think - and Allah knows best - is that a man gives a pledge to somebody in security for something. The pledge is superior to that for which he pawned it. The pledger says to the pawn-broker, 'I will bring you your due, after such-and-such a time. If not, the pledge is yours for what it was pawned for.' "
Malik said, "This transaction is not good and it is not halal. This is what was forbidden. If the owner brings what he pledged it for after the period, it is his. I think that the time condition is void."

قَالَ يَحْيَى حَدَّثَنَا مَالِكٌ، عَنِ ابْنِ شِهَابٍ، عَنْ سَعِيدِ بْنِ الْمُسَيَّبِ، أَنَّ رَسُولَ اللَّهِ ﷺ قَالَ " لاَ يَغْلَقُ الرَّهْنُ " .