What ruling applies to the marriage contract concluded by a guardian for a girl who is under ten years old, as exemplified by the case of Umm Kulthum bint Abi Bakr?

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Umar (may Allah be pleased with him) sought the hand of Umm Kulthum, daughter of Abu Bakr, in marriage after Abu Bakr's death, addressing Aisha. Aisha agreed while Umm Kulthum was less than ten years old, as she was born after her father's death. Umar's guardianship (Walayah) lasted ten years. When the minor disliked the match, Talha ibn 'Ubayd Allah married her, and no one objected. This demonstrates the consensus that her marriage before attaining puberty, under the guardianship of someone other than her father, is valid.