Ibn Abi Yahya (may Allah have mercy on him) says: I asked Rabi'ah (may Allah have mercy on him): What is the least amount of dowry? He said: Whatever the families agree upon. I said: Even if it is one dirham? He said: Even if it is half a dirham. I said: And if it is less than that? He said: Even if it is a handful of wheat or grains of wheat.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصداق / 14394