Sayyiduna Abu Hurayrah and Sayyiduna Abu Sa'id al-Khudri (may Allah be pleased with them both) said: "The hand is to be cut off for four dirhams or more." The Shaykh said: "It is possible that they said this only when the exchange value of a quarter of a dinar had become four dirhams. Likewise, what we have narrated from Sayyiduna Umar (may Allah be pleased with him) and others about five (dirhams) may have been when the exchange rate changed. The basis for the minimum amount (nisab) is a quarter of a dinar, as the established Sunnah that has passed indicates."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب السرقة / 17197