It is narrated from Ata bin Abi Rabah (may Allah have mercy on him) that Ibn Abbas (may Allah be pleased with him) did not consider it objectionable for a person to break a voluntary fast, and he used to give examples for this, saying: "If a man performed seven circuits (of Tawaf) but did not complete them, then he will have the reward according to his intention; or if someone prayed one rak'ah but did not pray the second, then he will have the reward according to the intention for which he sought reward."
Hadith Referenceمسند الإمام الشافعي / كتاب الصيام / 637
Hadith Takhrijاسناده صحيح وعنعنة ابن جريج عن عطاء مقبولة : اخرجه البيهقي : 4 / 277 ۔ وفي المعرفة السنن والآثار له (2565) وعبد الرزاق (7767)۔