Muhammad bin Yahya bin Habban said that a boy was brought before Sayyiduna Umar (may Allah be pleased with him) who, in his poetry, had accused a girl of committing fornication with him. Sayyiduna Umar (may Allah be pleased with him) said: Look at him. When they looked, it was found that his pubic hair had not grown, so he (may Allah be pleased with him) waived the prescribed punishment (hadd) from him. Abu Ubayd said: Some narrate it from Sayyiduna Uthman (may Allah be pleased with him). Abu Ubayd said: Ibtahara means that he made the accusation "I did this with her" while he is lying in his claim; if he really did it, that is called ibtiyar.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحجر / 11320