It is narrated from Bara' bin Azib (may Allah be pleased with him) that he (peace and blessings be upon him) ordered that the rows be closed up well, "so that the passing of something like young lambs does not make gaps between you." Awlad al-hadhaf means black hairless goats; they were found in Yemen.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 5185