Yahya related to me from Malik that Humayd ibn Qays and Thawr ibnZayd adDili both informed him that the Messenger of Allah, may Allahbless him and grant him peace, (and one of them gave more detail thanthe other),saw a man standing in the sun. The Messenger asked, "What'swrong with him?" The people said, "He has vowed not to speak or toseek shade from the sun or to sit and to fast." The Messenger ofAllah, may Allah bless him and grant him peace, said, "Go and tell himto speak, seek shade, and sit, but let him complete his fast."Malik said, "I have not heard that the Messenger of Allah, mayAllah bless him and grant him peace, ordered the man in question to doany kaffara. The Messenger of Allah, may Allah bless him and grant himpeace, only ordered him to complete that in which there was obedienceto Allah and to abandon that in which there was disobedience toAllah."
USC-MSA web (English) reference: Book 22, Hadith 6
Arabic reference: Book 22, Hadith 1018