Sahih Bukhari | Volume 8 | Hadith Number 695
Narrated by Ibn 'Abbas
While the Prophet was delivering a sermon, he saw a man standing, so he asked about that man. They (the people) said, "It is Abu Israil who has vowed that he will stand and never sit down, and he will never come in the shade, nor speak to anybody, and will fast.'' The Prophet said, "Order him to speak and let him come in the shade, and make him sit down, but let him complete his fast."