Maliks Muwatta Book 022, Hadith Number 006.

Section : Vows Not Permitted in Disobedience to Allah.

Yahya related to me from Malik that Humayd ibn Qays and Thawr ibn Zayd adDili both informed him that the Messenger of Allah, may Allah bless him and grant him peace, (and one of them gave more detail than the other),saw a man standing in the sun. The Messenger asked, "What's wrong with him?" The people said, "He has vowed not to speak or to seek shade from the sun or to sit and to fast." The Messenger of Allah, may Allah bless him and grant him peace, said, "Go and tell him to speak, seek shade, and sit, but let him complete his fast."

Malik said, "I have not heard that the Messenger of Allah, may Allah bless him and grant him peace, ordered the man in question to do any kaffara. The Messenger of Allah, may Allah bless him and grant him peace, only ordered him to complete that in which there was obedience to Allah and to abandon that in which there was disobedience to Allah."