Ata' bin Abi Rabah (may Allah have mercy on him) narrates that Ibn Abbas (may Allah be pleased with them both) saw no harm in a person breaking a voluntary fast, and he would give examples of it: a man who began seven circuits but did not complete them has the reward of what he did; and one who prayed one rak'ah and did not pray another has a reward accordingly.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8352